bake: Use threads to read/write from/to the main arbitrator process. Since the GIL is released most of the time during blocking I/O operations, this should let the worker threads do more during that time.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1