Attempt to reply exactly once to all requests

This allows requesters to block for an "ack" or ErrorResponse if they see
fit. Without the non-error ack, there's no way to know whether an error
is going to arrive later. So this is good general practice to always
have a "yes I got it" reply even if for now we never send back an
error for a particular request.
Merge pull request #104 from sbt/wip/havocp-reply-all-the-things

Attempt to reply exactly once to all requests