Improve the cleaning mechanism of ShardNode

[ZuLiangWang]

Describe this problem We found that the failover mechanism of the HoraeDB cluster failed, and the shard was not migrated when the machine went down.

Steps to reproduce

• Make the etcd root path configuration in HoraeDB and HoraeMeta inconsistent.
• Shut down a HoraeDB node.

Additional Information

• Add drop ShardNode api to deal with some extreme situations.
• Add a new way to detect failed nodes, not only relying on etcd's lease event.
  1. Use a background thread to continuously detect failed nodes.
  2. Detect failed nodes through heartbeat.