At last, merge the perf harness.

This branch is just short of a year old, and has been indifferently maintained while on tight deadlines.  The code quality could be better.

That said, it's better to get it in there so it doesn't bit rot quite so quickly in the future.

It builds on the rtssh code (was a modification of it for a long time), so there are places where I didn't understand the intention of the rtssh code or just couldn't get it to work in my environment.  There's likely a lot of stuff that could be cleaned up or made less redundant.  Suggestions appreciated.

cc @jtuple @cmeiklejohn