DRTVWR-418: Make LLEventPumps an LLHandleProvider for LLEventPump.

LLEventPump's destructor was using LLEventPumps::instance() to unregister the
LLEventPump instance from LLEventPumps. Evidently, though, there are lingering
LLEventPump instances that persist even after the LLSingletonBase::deleteAll()
call destroys the LLEventPumps LLSingleton instance. These were resurrecting
LLEventPumps -- pointlessly, since a newly-resurrected LLEventPumps instance
can have no knowledge of the LLEventPump instance! Unregistering is
unnecessary!

What we want is a reference we can bind into each LLEventPump instance that
allows us to safely test whether the LLEventPumps instance still exists.
LLHandle is exactly that. Make LLEventPumps an LLHandleProvider and bind its
LLHandle in each LLEventPump's constructor; then the destructor can unregister
only when LLEventPumps still exists.