filer: rebuild peer metadata subscriptions after a master reconnect (#10648)

* filer: keep the existing peer subscription on a repeated add

A cluster node add for a peer that is already followed restarted the
subscription, dropping the metadata events between the two runs.

* master: tell a connecting client the current cluster membership

Cluster node updates are only broadcast to the clients connected at that
moment. A filer that lost its master stream while a peer came back never
learned about the peer, and stopped replicating its metadata for good.

* test: a filer joining the master learns about the filers already there

* test: a filer resubscribes to a peer that registered while it was disconnected

Runs the reported sequence against real processes: filer2 leaves, filer1
is paused and its master stream is broken, filer2 registers again, and
filer1 has to replicate from it after reconnecting.

weed/server/master_grpc_server.go

@@ -437,6 +437,14 @@ func (ms *MasterServer) KeepConnected(stream master_pb.Seaweed_KeepConnectedServ
 		}
 	}
 
+	// Cluster node changes are only broadcast to the clients connected at that
+	// moment, so a client that reconnects has to be told who is around now.
+	for _, update := range ms.Cluster.ListClusterNodeUpdates(cluster.FilerGroupName(req.FilerGroup), cluster.FilerType) {
+		if sendErr := stream.Send(update); sendErr != nil {
+			return sendErr
+		}
+	}
+
 	if initialLockRingUpdate := ms.initialLockRingUpdate(req.ClientType, req.FilerGroup); initialLockRingUpdate != nil {
 		if sendErr := stream.Send(initialLockRingUpdate); sendErr != nil {
 			return sendErr