mvcc: update minRev when watcher stays synced

Problem: during restore in watchableStore.Restore, synced watchers are moved to unsynced.
minRev will be behind since it's not updated when watcher stays synced.

Solution: update minRev

fixes: etcd-io#15271
Signed-off-by: Bogdan Kanivets <bkanivets@apple.com>
Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>

server/storage/mvcc/watchable_store.go

@@ -448,12 +448,15 @@ func (s *watchableStore) notify(rev int64, evs []mvccpb.Event) {
 			pendingEventsGauge.Add(float64(len(eb.evs)))
 		} else {
 			// move slow watcher to victims
-			w.minRev = rev + 1
 			w.victim = true
 			victim[w] = eb
 			s.synced.delete(w)
 			slowWatcherGauge.Inc()
 		}
+		// always update minRev
+		// in case 'send' returns true and watcher stays synced, this is needed for Restore when all watchers become unsynced
+		// in case 'send' returns false, this is needed for syncWatchers
+		w.minRev = rev + 1
 	}
 	s.addVictim(victim)
 }