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.