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.

.github/workflows/metadata-subscribe-tests.yml

@@ -8,6 +8,7 @@ on:
       - 'weed/pb/filer_pb/**'
       - 'weed/util/log_buffer/**'
       - 'weed/server/filer_grpc_server_sub_meta.go'
+      - 'weed/server/master_grpc_server.go'
       - 'weed/command/filer_backup.go'
       - 'test/metadata_subscribe/**'
       - '.github/workflows/metadata-subscribe-tests.yml'
@@ -18,6 +19,7 @@ on:
       - 'weed/pb/filer_pb/**'
       - 'weed/util/log_buffer/**'
       - 'weed/server/filer_grpc_server_sub_meta.go'
+      - 'weed/server/master_grpc_server.go'
       - 'weed/command/filer_backup.go'
       - 'test/metadata_subscribe/**'
       - '.github/workflows/metadata-subscribe-tests.yml'