tor-rtmock: Provide a facility for handling other threads

See #1835 (closed).

This facility is intended to be useable for both "cpubound activity threads" and "do sync blocking io from async context". Currently it's not used anywhere but a test case.

This new facility makes it possible to overhaul BlockOn and SpawnBlocking as discussed in #1835 (closed), and implement the new traits properly for MockExcutor.

CC @wesleyac, who may want to steal review.