grpcproxy: fix race between watch ranges delete() and broadcasts empty()

Checking empty() wasn't grabbing the broadcasts lock so the race detector
flags it as a data race with coalesce(). Instead, just return the number
of remaining watches following delete() and get rid of empty().

proxy/grpcproxy/watch_broadcasts.go

@@ -96,7 +96,8 @@ func (wbs *watchBroadcasts) add(w *watcher) {
 	wbs.bcasts[wb] = struct{}{}
 }
 
-func (wbs *watchBroadcasts) delete(w *watcher) {
+// delete removes a watcher and returns the number of remaining watchers.
+func (wbs *watchBroadcasts) delete(w *watcher) int {
 	wbs.mu.Lock()
 	defer wbs.mu.Unlock()
 
@@ -110,10 +111,9 @@ func (wbs *watchBroadcasts) delete(w *watcher) {
 		delete(wbs.bcasts, wb)
 		wb.stop()
 	}
+	return len(wbs.bcasts)
 }
 
-func (wbs *watchBroadcasts) empty() bool { return len(wbs.bcasts) == 0 }
-
 func (wbs *watchBroadcasts) stop() {
 	wbs.mu.Lock()
 	for wb := range wbs.bcasts {

proxy/grpcproxy/watch_ranges.go

@@ -53,8 +53,7 @@ func (wrs *watchRanges) delete(w *watcher) {
 	if !ok {
 		panic("deleting missing range")
 	}
-	wbs.delete(w)
-	if wbs.empty() {
+	if wbs.delete(w) == 0 {
 		wbs.stop()
 		delete(wrs.bcasts, w.wr)
 	}