[Ballista] Refactor ballista scheduler server

[yahoNanJing]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently the ballista scheduler mod messes everything up in the lib file, which is not good for code reviewing.

Describe the solution you'd like

It's better to reorganize the scheduler contents as following:

• external_scaler, for exposing scheduler internal metrics
• grpc, for grpc service
• event_loop, for dealing with internal event-based state changes (TODO) [Ballista] Refactor the event channel #1909

Additional context
#1704