Allows callers to keep around a reference to an Avatar object and get
updated position and validity information without having to poll the
ObjectManager itself.
Put an abstract session and region implementation in client so things
that could be logically shared between client/proxy can be.
ObjectManager moved to client with proxy-specific details in
ProxyObjectManager.
The previous model didn't really map to how Indra handles objects:
In Indra Local IDs are only really used to look up the FullID, and
that's used to look at a global object list.
This moves to a model where the world (Session) owns the object, and
objects can be freely moved between regions without killing the world's
reference to it.
The two ID design for objects was a mistake in my opinion, but whatever.
Without this we end up in weird cases where the viewer gets a cache
hit and never request the object data, creating link heirarchies where
the viewer knows about all the prims but Hippolyzer only knows some
of them and orphans them.
Since we don't know what viewer the user is using, we scan around
the disk for object caches and try to use those. 99% of the time the
connection will be coming from localhost so this is fine.
Fixes#11
