Add test to demonstrate semantics of gRPC clients when server gets shutdown and client never calls Finish()

[benh]

We currently have tests to show that a gRPC client does not need to call Finish() but can simply get destructed.

It appears that this does not work when the server gets destructed, however, and that a client must call Finish() in that case, i.e., it can't just get destructed. We should write a test that demonstrates this as well as consider adding some functionality into our client code that makes this better.

[benh]

@while-false