storage: add watch ID to identify watchings

One watcher includes multiple watchings, and their events are
sent out through one channel. For the received event, user would like to
know which watching it belongs to.

Introduce a watch ID. When watching on some key, user will get a watch
ID. The watch ID is attached to all events that is observed by this
watch.

storage/watcher.go

@@ -26,7 +26,9 @@ type Watcher interface {
 	// The whole event history can be watched unless compacted.
 	// If `prefix` is true, watch observes all events whose key prefix could be the given `key`.
 	// If `startRev` <=0, watch observes events after currentRev.
-	Watch(key []byte, prefix bool, startRev int64) CancelFunc
+	// The returned `id` is the ID of this watching. It appears as WatchID
+	// in events that are sent to this watching.
+	Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc)
 
 	// Chan returns a chan. All watched events will be sent to the returned chan.
 	Chan() <-chan storagepb.Event
@@ -42,21 +44,27 @@ type watcher struct {
 	ch        chan storagepb.Event
 
 	mu      sync.Mutex // guards fields below it
+	nextID  int64      // nextID is the ID allocated for next new watching
 	closed  bool
 	cancels []CancelFunc
 }
 
 // TODO: return error if ws is closed?
-func (ws *watcher) Watch(key []byte, prefix bool, startRev int64) CancelFunc {
-	_, c := ws.watchable.watch(key, prefix, startRev, ws.ch)
+func (ws *watcher) Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc) {
 	ws.mu.Lock()
 	defer ws.mu.Unlock()
 	if ws.closed {
-		return nil
+		return -1, nil
 	}
+
+	id = ws.nextID
+	ws.nextID++
+
+	_, c := ws.watchable.watch(key, prefix, startRev, id, ws.ch)
+
 	// TODO: cancelFunc needs to be removed from the cancels when it is called.
 	ws.cancels = append(ws.cancels, c)
-	return c
+	return id, c
 }
 
 func (ws *watcher) Chan() <-chan storagepb.Event {