Subprocess runner singleton

Wrap all subprocess calls in a singleton that manages them

- Allow a queue that (async blocks) until finished, and thus have a configurable max number of subprocesses (e.g. 1 per core)

- Requires some experimentation in how asyncio waits for subprocesses, etc, along with some thinking about how to properly use cores

- Maybe merge this with subprocess asyncio stuff?

Start by locating all subprocess call sites and reviewing how asyncio currently waits for subprocesses. Experiment with a singleton queue that limits concurrent subprocesses, then verify that queued async blocks finish correctly and the concurrency limit can be configured.

Written by the indexing model from the issue text.