Add support for StartNoAckMode. Resolves #113.

For some reason, ack'ing responses was causing degenerate socket
behavior in either gdb or rr.  Omitting ack's speeds things up *a
lot*.  The unit tests run about 3.5x faster, which leads me to guess
that the gdb interactions are sped up around 5-6x.
Merge pull request #132 from cgjones/noack-mode

Add support for StartNoAckMode. Resolves #113.