Better handling of objects not known to the proxy due to viewer object cache hits

[SaladDais]

Whenever the viewer enters a sim, the sim will send cache probes with ObjectUpdateCached containing a LocalID and CRC. If the LocalID isn't cached or there's a CRC mismatch the viewer will request it with RequestMultipleObjects. Right now the proxy doesn't know about any objects that the viewer didn't request due to them being present in its own viewer object cache.

Right now we hack around this in a few places by always requesting missing objects when they're referenced in an ObjectSelect packet, but it can cause problems if you're trying to look at avatar positions and the avatar sits on an object the proxy doesn't know about. Object positions are always relative to their parent, so not knowing the parent's position makes it hard to determine the avatar's position.

Possible approaches:

• Always send VOCACHE_IS_EMPTY flag in RegionHandshakeReply so cache probes don't get sent, and the sim will always send full ObjectUpdates instead. This is the least brittle, but most wasteful because the sim always has to send ObjectUpdates for objects your viewer may have cached.
• Parse the viewer's vocache and pull objects into ObjectManager on ObjectUpdateCached LocalID + CRC match. Might be brittle, and I don't know if vocache supports concurrent reads & writes well. Discovering viewer cache locations is also a problem.
• Have the proxy maintain it's own vocache. Least likely to cause problems with the viewer, but would involve the most code for cache bookkeeping.

[SaladDais]

9 would help with approach 2 since the viewer object cache is just a bunch of ObjectUpdateCompressed Data sections in files keyed on region coordinates.

VOCache is only written when leaving a region, so mapping across all viewers' VOCaches and looking for a (region_handle, local_id, crc) match would work fine for our purposes with relatively unlikely race conditions.