Now the port is not part of the API, there's a method to close it,
__init__ can take a string in addition to the objects it used to take,
and the class is a context manager for the with statement.  I can
confidently say that these are all great changes.

It returns the SinkConfig in the standard format, just like the serial
configuration interface.

The timeout was a dirty hack from the start.  It was long enough that
commands have finished running by the time it completes, but it wasn't
the right way to see when a command finished running.  The right way is
to look for the PDBS) prompt, which is what we do now.

This makes the Sink.set_tmpcfg method run noticeably faster.  Neat.