Refactored host_callback to use the C++ runtime. (#3644)
* Refactored host_callback to use the C++ runtime.
* The new runtime makes it unnecessary to start the outfeed_receiver
in the user's code
* We don't need msgpack anymore
* There is an interaction between host_callback and using lax.outfeed.
I am trying to solve this by (a) making host_callback_test stop the
outfeed receiver on finish and infeed_test on start, and (b)
telling pytest-xdist to run all the tests from one file into
a single worker.
