The GH actions CI hangs were due to a deadlock in rt_spawn.
The GH runners have only two cores available which meant
that it would only take one call to send_blocking or recv_blocking
to tie up the localset threads and deadlock.
This commit adjusts that logic to make rt_spawn async and to
provide a non-blocking fire and forget variant for the couple
of situations that need it.
Also adds a bit of tracing!
This has our first end-to-end integration test that validates
smtp -> source mta -> sink mta -> maildir
and confirms that the message has the right bits inside it.
The end to end test will get refactored into more easily usable
pieces in follow-up commit(s).
Wez Furlong