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_batch_delete.go

@@ -11,9 +11,12 @@ import (
 )
 
 func (vs *VolumeServer) BatchDelete(ctx context.Context, req *volume_server_pb.BatchDeleteRequest) (*volume_server_pb.BatchDeleteResponse, error) {
-
 	resp := &volume_server_pb.BatchDeleteResponse{}
 
+	if err := vs.CheckMaintenanceMode(); err != nil {
+		return resp, err
+	}
+
 	now := uint64(time.Now().Unix())
 
 	for _, fid := range req.FileIds {