watchtower: update DeleteCommittedUpdate to delete all

This commit updates the DeleteCommittedUpdate DB method to delete all of
a given session's committed updates instead of just one at a time. The
reason for this is that in an upcoming commit, we will introduce a
"Terminal" session state - once we have deleted a committed update for a
session it should be considered "Terminal" and there is never a case
where we would only want to delete one committed update and not the
rest. So we want these two actions (deleting committed updates of a
session and setting it's status to terminal) to be atomic.

watchtower/wtclient/session_queue.go

@@ -211,14 +211,13 @@ func (q *sessionQueue) Stop(final bool) error {
 					update.BackupID, err)
 				continue
 			}
+		}
 
-			err = q.cfg.DB.DeleteCommittedUpdate(
-				q.ID(), update.SeqNum,
-			)
+		if final {
+			err = q.cfg.DB.DeleteCommittedUpdates(q.ID())
 			if err != nil {
 				log.Errorf("could not delete committed "+
-					"update %d for session %s",
-					update.SeqNum, q.ID())
+					"updates for session %s", q.ID())
 			}
 		}