New PyMoose runtime that connects directly to workers using gRPC

[mortendahl]

This is a discussion and exploration issue. The target use case is OSS where someone wants to drive everything directly from Python instead of eg using FileChoreography as in Rudolph.

While not directly part of this issue, keep in mind that this is also a step in the direction of having Python REPL support for Moose, where outputs from one computation can be used as inputs to the next. As a first step this could be done (insecurely) by sending all outputs back to Python, but goal is to have values stay on the remove machines.