[FLINK-9669] Add assignment store interface.

[liurenjie1024]

What is the purpose of the change

This pull requests is part of process to enable task manager isolation in flink 1.5 session mode.

Brief change log

• Add task manager assignment class.
• Add task manager assignment store interface.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving):no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: yes
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? JavaDocs

[tillrohrmann]

For what do we need the assignment store interface if we go with the tag based scheduling solution @liurenjie1024?

[liurenjie1024]

@tillrohrmann This is from my initial design, and since the design has changed, we can close this now.