Since we can now create and destroy pg pools, the pool ids will be sparse,
and an array no longer makes sense for looking up by pool id.  Use an
rbtree instead.

The OSDMap encoding also no longer has a max pool count (previously used to
allocate the array).  There is a new pool_max, that is the largest pool id
we've ever used, although we don't actually need it in the client.
Signed-off-by: NSage Weil <sage@newdream.net>