Block RPC write operations on volume servers when maintenance mode is enabled (#8115)

* Boostrap persistent state for volume servers.

This PR implements logic load/save persistent state information for storages
associated with volume servers, and reporting state changes back to masters
via heartbeat messages.

More work ensues!

See https://github.com/seaweedfs/seaweedfs/issues/7977 for details.

* Block RPC operations writing to volume servers when maintenance mode is on.

weed/server/volume_grpc_remote.go

@@ -15,6 +15,10 @@ import (
 )
 
 func (vs *VolumeServer) FetchAndWriteNeedle(ctx context.Context, req *volume_server_pb.FetchAndWriteNeedleRequest) (resp *volume_server_pb.FetchAndWriteNeedleResponse, err error) {
+	if err := vs.CheckMaintenanceMode(); err != nil {
+		return nil, err
+	}
+
 	resp = &volume_server_pb.FetchAndWriteNeedleResponse{}
 	v := vs.store.GetVolume(needle.VolumeId(req.VolumeId))
 	if v == nil {