clientv3: close open watch channel if substream is closing on reconnect

If substream is closing but outc is still open while reconnecting, then outc
would only be closed once the watch client would connect or once the watch
client is closed. This was leading to deadlocks in the proxy tests. Instead,
close immediately if the context is canceled.

Fixes etcd-io#7503

clientv3/watch.go

@@ -694,6 +694,10 @@ func (w *watchGrpcStream) waitCancelSubstreams(stopc <-chan struct{}) <-chan str
 		go func(ws *watcherStream) {
 			defer wg.Done()
 			if ws.closing {
+				if ws.initReq.ctx.Err() != nil && ws.outc != nil {
+					close(ws.outc)
+					ws.outc = nil
+				}
 				return
 			}
 			select {