Better interface compatibility with `concurrent.futures`

PyCCC engines are basically `Executor`s, jobs are basically `Future`s, etc. Should get as close to the `concurrent.futures` abstract interfaces as possible.

Also use more common terminology when possible - probably replace `runs_remotely` with `docker_rpc` or similar.

Start by locating the PyCCC engine and job entry points, then compare their interfaces with Python's concurrent.futures Executor and Future abstractions. Inventory the existing runs_remotely terminology and determine the compatibility and naming scope; done requires an agreed, consistent interface and terminology plan.

Written by the indexing model from the issue text.