Granular Worker management

[eirrgang]

Ref #335

• [x] Move dispatch out of WorkflowManager.
• [x] Move executor factory out of WorkflowManager responsibilities
• [ ] Make sure WorkflowManager context is re-usable.
• [ ] Rename "dispatch" -> "executor"
• [ ] Solidify Executor role with a single implementation to manage the dispatching context: Merge Queuer and manage_execution
• [ ] Implement Executor in terms of CPI/RPC spec to be specialized by RuntimeManagers. Executor and RuntimeManager collaborate to fulfill Futures.
• [ ] Call "stop" as part of ordinary shutdown.
• [ ] Add a CPI command to start/stop Worker.
• [ ] Separate the Worker lifetime from master lifetime.
• [ ] Launch Worker according to work load.

[codecov[bot]]

Codecov Report

Patch coverage: 58.06% and project coverage change: -0.28 :warning:

Comparison is base (d4d428c) 80.55% compared to head (082e966) 80.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #355 +/- ## ========================================== - Coverage 80.55% 80.27% -0.28% ========================================== Files 30 30 Lines 3399 3453 +54 Branches 518 530 +12 ========================================== + Hits 2738 2772 +34 - Misses 481 498 +17 - Partials 180 183 +3 ``` | [Impacted Files](https://app.codecov.io/gh/SCALE-MS/scale-ms/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SCALE-MS) | Coverage Δ | | |---|---|---| | [src/scalems/radical/runtime.py](https://app.codecov.io/gh/SCALE-MS/scale-ms/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SCALE-MS#diff-c3JjL3NjYWxlbXMvcmFkaWNhbC9ydW50aW1lLnB5) | `76.30% <58.06%> (-0.99%)` | :arrow_down: |

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