Having a map from std::string to a factory function returning LLEventPump* is
a cool idea, especially since you could statically populate such a map with
string literals and little lambdas.

Unfortunately, static initialization of any data is a bad idea when control
can reach consuming code before that module's static data are constructed.

Since LLEventPumps is already an LLSingleton, it's simple enough to make its
map non-static and initialize it in the constructor.

But another recent static factory-function map was introduced in
llleaplistener.cpp to support the LLLeapListener::newpump() operation. That
involves no LLSingletons.

Introduce LLEventPumps::make(name, tweak, type) to instantiate an LLEventPump
subclass of the specified type with specified (name, tweak) parameters.
Instances returned by make() are owned by LLEventPumps, as with obtain().
Introduce LLEventPumps::BadType exception for when the type string isn't
recognized.

LLEventPumps::obtain() can then simply call make() when the specified instance
name doesn't already exist. The configuration data used internally by obtain()
becomes { string instance name, string subclass name }. Although this too is
currently initialized in the LLEventPumps constructor, migrating it to a
configuration file would now be far more straightforward than before.

LLLeapListener::newpump(), too, can call LLEventPumps::make() with the
caller-specified type string. This eliminates that static factory map.
newpump() must catch BadType and report the error back to its invoker.

Given that the LLEventPump subclass instances returned by make() are owned by
LLEventPumps rather than LLLeapListener, there is no further need for the
LLLeapListener::mEventPumps ptr_map, which was used only to manage lifetime.
Also remove LLLeapListener's "killpump" operation since LLEventPumps provides
no corresponding functionality.

No one uses LLEventQueue to defer posted events until the next mainloop tick
-- and with LLCoros moving to Boost.Fiber, cross-coroutine event posting works
that way anyway, making LLEventQueue pretty unnecessary.

The static RegisterFlush instance in llevents.cpp was used to call
LLEventPumps::flush() once per mainloop tick, which in turn called flush() on
every registered LLEventPump. But the only reason for that mechanism was to
support LLEventQueue. In fact, when LLEventMailDrop overrode its flush()
method for something quite different, it was startling to find that the new
flush() override was being called once per frame -- which caused at least one
fairly mysterious bug. Remove RegisterFlush. Both LLEventPumps::flush() and
LLEventPump::flush() remain for now, though intended usage is unclear.

Eliminating LLEventQueue means we must at least repurpose
LLEventPumps::mQueueNames, a map intended to make LLEventPumps::obtain()
instantiate an LLEventQueue rather than the default LLEventPump. Replace it
with mFactories, a map from desired instance name to a callable returning
LLEventPump*. New map initialization syntax plus lambda support allows us to
populate that map at compile time with little lambdas returning the correct
subclass instance.

Similarly, LLLeapListener::newpump() used to check the ["type"] entry in the
LLSD request specifically for "LLEventQueue". Introduce another such map in
llleaplistener.cpp for potential future extensibility.

Eliminate the LLEventQueue-specific test.

The previous implementation went to some effort to crash if anyone attempted
to create or destroy an LLInstanceTracker subclass instance during traversal.
That restriction is manageable within a single thread, but becomes unworkable
if it's possible that a given subclass might be used on more than one thread.

Remove LLInstanceTracker::instance_iter, beginInstances(), endInstances(),
also key_iter, beginKeys() and endKeys(). Instead, introduce key_snapshot()
and instance_snapshot(), the only means of iterating over LLInstanceTracker
instances. (These are intended to resemble functions, but in fact the current
implementation simply presents the classes.) Iterating over a captured
snapshot defends against container modifications during traversal. The term
'snapshot' reminds the coder that a new instance created during traversal will
not be considered. To defend against instance deletion during traversal, a
snapshot stores std::weak_ptrs which it lazily dereferences, skipping on the
fly any that have expired.

Dereferencing instance_snapshot::iterator gets you a reference rather than a
pointer. Because some use cases want to delete all existing instances, add an
instance_snapshot::deleteAll() method that extracts the pointer. Those cases
used to require explicitly copying instance pointers into a separate
container; instance_snapshot() now takes care of that. It remains the caller's
responsibility to ensure that all instances of that LLInstanceTracker subclass
were allocated on the heap.

Replace unkeyed static LLInstanceTracker::getInstance(T*) -- which returned
nullptr if that instance had been destroyed -- with new getWeak() method
returning std::weak_ptr<T>. Caller must detect expiration of that weak_ptr.

Adjust tests accordingly.

Use of std::weak_ptr to detect expired instances requires engaging
std::shared_ptr in the constructor. We now store shared_ptrs in the static
containers (std::map for keyed, std::set for unkeyed).

Make LLInstanceTrackerBase a template parameterized on the type of the static
data it manages. For that reason, hoist static data class declarations out of
the class definitions to an LLInstanceTrackerStuff namespace.

Remove the static atomic sIterationNestDepth and its methods incrementDepth(),
decrementDepth() and getDepth(), since they were used only to forbid creation
and destruction during traversal.

Add a std::mutex to static data. Introduce an internal LockStatic class that
locks the mutex while providing a pointer to static data, making that the only
way to access the static data.

The LLINSTANCETRACKER_DTOR_NOEXCEPT macro goes away because we no longer
expect ~LLInstanceTracker() to throw an exception in test programs.
That affects LLTrace::StatBase as well as LLInstanceTracker itself.

Adapt consumers to the new LLInstanceTracker API.

Every LEAP plugin gets its own LLLeapListener, managing its own collection of
listeners to various LLEventPumps. LLLeapListener's command LLEventPump now
has a UUID for a name, both for uniqueness and to make it tough for a plugin
to mess with any other.