grpcproxy: fix race between coalesce and bcast on nextrev

coalesce was locking the target coalesce broadcast object but not the source
broadcast object resulting in a data race on the source's nextrev.

proxy/grpcproxy/watch_broadcasts.go

@@ -59,6 +59,7 @@ func (wbs *watchBroadcasts) coalesce(wb *watchBroadcast) {
 		if wbswb == wb {
 			continue
 		}
+		wb.mu.Lock()
 		wbswb.mu.Lock()
 		// 1. check if wbswb is behind wb so it won't skip any events in wb
 		// 2. ensure wbswb started; nextrev == 0 may mean wbswb is waiting
@@ -71,6 +72,7 @@ func (wbs *watchBroadcasts) coalesce(wb *watchBroadcast) {
 			wb.receivers = nil
 		}
 		wbswb.mu.Unlock()
+		wb.mu.Unlock()
 		if wb.empty() {
 			delete(wbs.bcasts, wb)
 			wb.stop()