Fixes to ensure IPC (channel) breakage doesn't result in hung actor
trees; the zombie reaping and general supervision machinery will always
clean up and terminate.

This includes not only the (mostly minor) fixes to solve these cases but
also a new extensive test suite in `test_advanced_faults.py` with an
accompanying highly configurable example module-script in
`examples/advanced_faults/ipc_failure_during_stream.py`. Tests ensure we
never get hang or zombies despite operating in debug mode and attempt to
simulate all possible IPC transport failure cases for a local-host actor
tree.

Further we simplify `Context.open_stream.__aexit__()` to just call
`MsgStream.aclose()` directly more or less avoiding a pure duplicate
code path.
