Merge changes from topic "snapshotctl_cleanup" into rvc-dev

* 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.
  snapshotctl: init reports merge statistics
  libsnapshot/test: Re-enable the failing tests