Join ObjectStorage and WritableObjectStorage interfaces

We had ObjectLocator (base ObjectManager class) and DbStorage (backend storage)
implementing the same ObjectStorage interface, this doesn't really make sense
as the query options are different, additionally backend returns rows and
the frontend returns domain models.  This is basically a violation of LSP.

Make ObjectLocator no longer implement a particular interface, merge
WritableObjectStorage and ObjectStorage, and make DbStorage implement
ObjectStorage.

Also updated a few class comments.

Change-Id: I9962ff4b4e4aa42d2d5e210eab333bbbfcf2e754
Merge "Join ObjectStorage and WritableObjectStorage interfaces"