Revert "Revert "Utilize BufferedCache in TreeRepository""

This reverts commit 50dbdb9fb597f342b356209a9fbc82cfa1f5b37f.

When a new revision fails to commit both BufferedCache and the Database
are rolled back, unfortunatly TreeRepository is using a plain
BagOStuff so we can have a situation where a post exists in the
TreeRepository memcache but not in the actual database.

There was some problem with this patch before that caused us to
revert it, but noone remembers what it is.  Please test or
try and remember so we can move this forward.

Change-Id: I9ff907e23f4c4e8f8afc5672f1a8f55fed08ea19
Merge "Revert "Revert "Utilize BufferedCache in TreeRepository"""