Also a little review of Dead objects in general.
The old code was calling 'erase' on a large vector, once for
every dead object (which are a lot, especially if you exit or
teleport), causing the whole (large) vector to be copied every
time. The new code only calls erase once per call (about once
a second at most now), erasing 20 to 100 objects at the END
of the vector. This is INCREDIBLY faster.
Added LLObjectList::getAvatar(LLUUID) for type-safe and quicker lookup
Added F32 overload to LLSD because using F64 needlessly is silly.
(And its lack of F32 overload caused issues with LLCachedControl<F32>)
