Given the viewer's mutually-dependent LLSingletons, given that different
threads might simultaneously request different LLSingletons from such a chain
of circular dependencies, the key to avoiding deadlock is to serialize all
LLSingleton construction on one thread: the main thread. Add comments to
LLSingleton::getInstance() explaining the problem and the solution.

Recast LLSingleton's static SingletonData to use LockStatic. Instead of using
Locker, and simply trusting that every reference to sData is within the
dynamic scope of a Locker instance, LockStatic enforces that: you can only
access SingletonData members via LockStatic.

Reorganize the switch in getInstance() to group the CONSTRUCTING error, the
INITIALIZING/INITIALIZED success case, and the DELETED/UNINITIALIZED
construction case.

When [re]constructing an instance, on the main thread, retain the lock and
call constructSingleton() (and capture_dependency()) directly.

On a secondary thread, unlock LockStatic and use LLMainThreadTask::dispatch()
to call getInstance() on the main thread. Since we might end up enqueuing
multiple such tasks, it's important to let getInstance() notice when the
instance has already been constructed and simply return the existing pointer.

Add loginfos() method, sibling to logerrs(), logwarns() and logdebugs().
Produce loginfos() messages when dispatching to the main thread, when actually
running on the main thread and when resuming the suspended requesting thread.

Make deleteSingleton() manage all associated state, instead of delegating some
of that work to ~LLSingleton(). Now, within LockStatic, extract the instance
pointer and set state to DELETED; that lets subsequent code, which retains the
only remaining pointer to the instance, remove the master-list entry, call the
subclass cleanupSingleton() and destructor without needing to hold the lock.

In fact, entirely remove ~LLSingleton().

Import LLSingletonBase::cleanup_() method to wrap the call to subclass
cleanupSingleton() in try/catch.

Remove cleanupAll() calls from llsingleton_test.cpp, and reorder the success
cases to reflect the fact that T::cleanupSingleton() is called immediately
before ~T() for each distinct LLSingleton subclass T.

When getInstance() on a secondary thread dispatches to the main thread, it
necessarily unlocks its LockStatic lock. But an LLSingleton dependency chain
strongly depends on the function stack on which getInstance() is invoked --
the task dispatched to the main thread doesn't know the dependencies tracked
on the requesting thread stack. So, once the main thread delivers the instance
pointer, the requesting thread captures its own dependencies for that
instance.

Back in the requesting thread, obtaining the current EInitState to pass to
capture_dependencies() would have required relocking LockStatic. Instead, I've
convinced myself that (a) capture_dependencies() only wanted to know
EInitState to produce an error for CONSTRUCTING, and (b) in CONSTRUCTING
state, we never get as far as capture_dependencies() because getInstance()
produces an error first.

Eliminate the EInitState parameter from all capture_dependencies() methods.
Remove the LLSingletonBase::capture_dependency() stanza that tested
EInitState. Make the capture_dependencies() variants that accepted LockStatic
instead accept LLSingletonBase*. That lets getInstance(), in the
LLMainThreadTask case, pass the newly-returned instance pointer.

For symmetry, make pop_initializing() accept LLSingletonBase* as well, instead
of accepting LockStatic and extracting mInstance.