[FileBackend] MultiWrite code improvements and sanity checks.

[FileBackend] MultiWrite code improvements and sanity checks.

* Automatically override a few sub-backend settings in FileBackendMultiWrite.
  We should ignore locking, journaling, or read-only settings of sub-backends.
  Journaling is now done just for master sub-backend paths (under the proxy backend name).
  We don't need to log for each sub-backend, which is a waste of disk space.
* Changed FileBackendMultiWrite::doOperationsInternal() to be similiar to the
  other write functions. It now uses doOperations() of the sub-backends rather
  than manual code. This makes settings like 'concurrency' easier to manage;
  we might want to have some sub-backends with varying setings for that.
* Made FileBackendMultiWrite::doQuickOperationsInternal() compliant with docs.
* Added CHECK_SHA1 option for consistency checking.
* Fixed function visibility in two places.
* Improved multiwrite backend tests by calling consistencyCheck().

Change-Id: Iac7bfe10c77ecd069fb9ef0ec26a01512f5f4eea