Both have to do with the test runner's handling of the tests. With one
it's the read thread somehow outliving the other threads (e.g. with
ctrl-C). The other is due to a filled OS-level pipe's buffer causing a
block while code is still holding the GIL in some gRPC core function. We
can't empty the buffer from Python because the GIL is held, and the OS
can't unblock because it's waiting for the buffer to get cleared:
deadlock.

Revert "Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()""

Move C-layer call creation into GRPCChannel, so that it always acts on a
non-destroyed channel.

Listen for connectivity in the GRPCCall, and destroy the channel.

- Revised 'protoc' section and 'Documentation' section as LisaFC suggested.
- Added 'Examples' section as LisaFC suggested.

Resolves: #5661