Set a more obvious prefix for the threads in the gRPC ThreadPoolExecutor

At the moment our logs are full of thread names like ThreadPoolExecutor-0_100. That doesn't give a whole lot of useful information without extra knowledge of where and how exactly we use a ThreadPoolExecutor.

This MR passes a prefix to the ThreadPoolExecutor, so that we now produce log lines like gRPC_Executor_100, which gives more context on what the various threads are actually for at a glance.