Consider using asyncio executor for gRPC

An asyncio grpc server executor was implemented and merged in the upstream grpc repo: https://github.com/grpc/grpc/pull/20598

BuildGrid should start using this executor, however, it won't be a trivial change. Here is discussion in the mailing list highlighted this issue: https://lists.buildgrid.build/pipermail/buildgrid/2019-January/000120.html

Full rewrite using this executor, changing to a single-threaded, event loop driven server implementation.

Edited Mar 03, 2021 by Adam Coldrick