Feature: [metasrv]: add API watch

[drmingdrmer]

Summary

A client requests metasrv to create a stream to receive events about a key.

A good reference of watching API is: etcd-v3 watch API: https://etcd.io/docs/v3.2/learning/api/#watch-api

• Add grpc API to create watch stream to meta.proto:

    rpc Watch(WatchRequest) returns (stream WatchReply) {}
  

• When metasrv receives a WatchRequest, it subscribes to a queue for apply events.
• Add callback to state machine: every time SM applied an entry, feed the prev/result to a queue. #4607

[drmingdrmer]

TODO: split into sub issues

[flaneur2020]

can we take advantage of the Watch API to make a distributed lock? maybe there are cases like a distributed task (https://docs.snowflake.com/en/sql-reference/sql/create-task.html), each task has to be executed ONCE in a query instance, avoiding two query instances get executed in the same time. thus the tasks can be protected by a distributed lock.

[drmingdrmer]

Yes, a lock can be implemented with watch.

But the execute-exactly-once semantic can not be implemented with just a lock. You still need some stateful service to store if the task has already been executed.

[BohuTANG]

This issue seems can be closed after #4779 @drmingdrmer