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.