Implement MetaStorageManager component stop

MetaStorageManager stop mechanism should be implemented. Resources, that should be stopped or cleared during stop:

• MetaStorageServiceImpl$WatchProcessor$Watcher threads should be stopped properly
• RaftGroupService should be shutdown
• Meta storage raft group should be stopped if the node hosts meta storage
  All MetaStorageManager public methods should be guarded by read lock, stop should be done using write lock.