Use the meta watch api to refactor some mod

[zhang2014]

Meta client supported the watch api, which makes it possible to subscribe to a key (instead of polling).

• Cluster
• Settings
• UserApi
• UserPrivilege

[drmingdrmer]

@drmingdrmer embedded meta-store needs to support watch API

[sundy-li]

@drmingdrmer embedded meta-store needs to support watch API

Is there a faked watch API for embedded meta ?

[drmingdrmer]

Embedded meta is a burden because two sets of API have to be maintained.
Now it's a good time to discuss getting rid of embedded meta.

[sundy-li]

We need to start this task now. Make settings support watch API based on meta-service.

[drmingdrmer]

I'll be getting on this when meta-service finished upgrading from openraft 0.7 to 0.8