Speed up of LLViewerObjectList::cleanDeadObjects with a factor of 10000.

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.

indra/newview/llviewerobjectlist.h

@@ -190,6 +190,7 @@ public:
 	S32 mNumDeadObjectUpdates;
 	S32 mNumUnknownKills;
 	S32 mNumDeadObjects;
+	S32 mMinNumDeadObjects;
 protected:
 	std::vector<U64>	mOrphanParents;	// LocalID/ip,port of orphaned objects
 	std::vector<OrphanInfo> mOrphanChildren;	// UUID's of orphaned objects