Added VolumeMarkWritable and VolumeStatus grpc methods

This is necessary for copy to mark as read-only and then restore the
original state afterwards.

weed/storage/store.go

@@ -307,7 +307,20 @@ func (s *Store) MarkVolumeReadonly(i needle.VolumeId) error {
 	if v == nil {
 		return fmt.Errorf("volume %d not found", i)
 	}
+	v.noWriteLock.Lock()
 	v.noWriteOrDelete = true
+	v.noWriteLock.Unlock()
+	return nil
+}
+
+func (s *Store) MarkVolumeWritable(i needle.VolumeId) error {
+	v := s.findVolume(i)
+	if v == nil {
+		return fmt.Errorf("volume %d not found", i)
+	}
+	v.noWriteLock.Lock()
+	v.noWriteOrDelete = false
+	v.noWriteLock.Unlock()
 	return nil
 }