netwk/ Expect(...) now sends CLIENT_ACKs

Whenever a message is consumed in an Expect(...) method we send a
CLIENT_ACK message for the message just consumed to prevent it from
being re-sent. We then wait for a timeout to ensure no new re-sends are
coming. This should prevent some raciness we were seeing in the netwk
testsuit where server re-sends were being picked up and causing test
failures.
Unit testing around responder resends

We now have unit testing around the resend code in the responder. These
are unit tests directly testing at a low method level. I felt that the
integration testing style would prove too clumsy for fully exercising
this feature.