Merge changes from topics "libsnapshot_delete_initiate",...

Merge changes from topics "libsnapshot_delete_initiate", "snapshotctl_cleanup", "update_engine_vab_metrics"

* changes:
  libsnapshot: SnapshotMergeStats::Start/Finish
  libsnapshot: Expose SnapshotMergeStats
  libsnapshot: delete WaitForMerge.
  libsnapshot: handle errors in RemoveAllUpdateState appropriately.
  libsnapshot: remove snapshots properly after flashing
  libsnapshot: RemoveUpdateState on rollback.
  libsnapshot: NeedSnapshotsInFirstStageMount don't test for IsRecovery
  Allow ProcessUpdateState to be paused.
  libsnapshot: Re-expose InitiateMerge and ProcessUpdateState
  libsnapshot: Add prolog to RemoveAllUpdateStates.
  snapshotctl don't auto-merge.