Create AthenaX Backend Core component with flexible DB storage / Instance Management / Cluster Management APIs

[walterddr]

Split out athenax-backend into more flexible component groups to enable

• APIs for replacing LevelDB with any database system

• APIs for replacing YARN with any cluster management and Flink deployment models (remove dependencies with YARN / Hadoop)

• APIs for replacing built-in watchdog policies with any instance management strategies.

• Higher Lever Server Context APIs for enabling or extending future functionalities

Attached design Doc: https://docs.google.com/document/d/1oWcYKz2knrRUVhnh6Xl53qD1sA88qSFBtBkPvX6KPM0/edit?usp=sharing

[walterddr]

close, change of plan