Add volume server RPCs to read and update state flags. (#8186)

* 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.

* Add volume server RPCs to read and update state flags.

weed/storage/store_state.go

@@ -69,3 +69,19 @@ func (st *State) Save() error {
 	glog.V(1).Infof("Saved store state %v to %s", st.Pb, st.FilePath)
 	return nil
 }
+
+func (st *State) Update(state *volume_server_pb.VolumeServerState) error {
+	if state == nil {
+		return nil
+	}
+
+	origState := st.Pb
+	st.Pb = state
+	err := st.Save()
+	if err != nil {
+		// restore the original state upon save failures, to avoid skew between in-memory and disk state protos.
+		st.Pb = origState
+	}
+
+	return err
+}