Controller Refactor Part 1: separate communication

[SYangster]

• separate out communication from controller with composition
• WFCommServer (closely coupled with Responder pattern)
• WFCommClient (uses aux channel -> will require new executor in follow up PR to support converting existing server-controlled workflows into client-controlled workflows)

Update: Responder: handle_dead_job() and ControllerSpec: process_result_of_unknown_task() are implemented by some Controller subclasses, but are needed by the WFCommServer. For now I passed the Controller to the WFCommServer, but maybe need a cleaner solution.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[SYangster]

/build

[YuanTingHsieh]

Mostly LGTM, added some comments.

[yanchengnv]

Very impressive! I like it that all existing controllers don't need changes (other than change some imports).
We need to make the WFCommSpec more complete (see my comments). I understand that most of these are only specific to the server side (for now), but the concept is good and may still be applicable to other implementations. The default impl of these methods can raise exception NOT_IMPLEMENTED.

[SYangster]

/build

[yanchengnv]

LGTM