Ie161e0f was done in a hurry, and so didn't do things in the best ways.
This introduces a new "CachedBagOStuff" that transparently handles all
the logic that had been copy-pasted all over in Ie161e0f.
The differences between CachedBagOStuff and MultiWriteBagOStuff are:
* CachedBagOStuff supports only one "backend".
* There's a flag for writes to only go to the in-memory cache.
* The in-memory cache is always updated.
* Locks go to the backend cache (with MultiWriteBagOStuff, it would wind
up going to the HashBagOStuff used for the in-memory cache).
Change-Id: Iea494729bd2e8c6c5ab8facf4c241232e31e8215
