Support per-BatchRefUpdate atomic transactions

Support per-BatchRefUpdate atomic transactions

Repurpose RefDatabase#performsAtomicTransactions() slightly, to
indicate that the backend _supports_ atomic transactions, rather than
the current definition, which is that the backend always _uses_ atomic
transactions regardless of whether or not the caller actually wants
them. Allow BatchRefUpdate callers to turn off atomic transactions by
calling setAtomic(false). Defaulting to true means this is backwards
compatible.

Change-Id: I6df78d7df65ab147b4cce7764bd3101db985491c