reftable: split off generic code from DFS code

reftable: split off generic code from DFS code

This introduces ReftableBatchRefUpdate and ReftableDatabase, as
generic classes, with some code moved to DfsReftableBatchRefUpdate and
DfsReftableDatabase.

Clarify thread-safety requirements by asserting locked status in
accessors, and acquiring locks in callers. This does not fix threading
problems, because ReftableBatchRefUpdate already wraps the whole
transaction in a lock.

This also fixes a number of bugs in ReftableBatchRefUpdate:

* non-atomic updates should not bail on first failure

* isNameConflicting should also check for conflicts between names that
  are added and removed in the BatchRefUpdate.

Change-Id: I5ec91173ea9a0aa19da444c8c0b2e0f4e8f88798
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>