`asyncio` (mode) debugger support

[goodboy]

Replacement for #351 to start effort toward testing our debugger (with pdbp) and basic functionality when running actors in infected_asyncio mode.

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Test set todo:

• [ ] ensure std breakpoint() calls work outside tractor.open_nursery()
  • [ ] we might want to ensure something something with threads and/or maybe numba?
• [ ] add a test suite for use inside an infected asyncio actor:
  • [ ] asyncio task uses breakpoint() directly, expect normal UX
  • [ ] asyncio task crashes and trio side enters debug? (or should the aio task get caught?)
  • [ ] cancel actor from parent and ensure that if some asyncio-task has debugger lock, normal teardown blocking is adhered?

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Any new issues?

• [ ] go through our list here: https://github.com/goodboy/tractor/issues?q=is%3Aissue+is%3Aopen+label%3Adebugger
  • [ ] of particular note is the hang issue recently found in https://github.com/goodboy/tractor/issues/354, which has an example / test added in #351