Feeble performance hack for the threaded RTS: instead of
allocating/releasing a new condition variable for each new call-in, we
just cache one in the RTS and re-use it for the next call.

On a little test I have here which does lots of call-ins on Windows,
this reduces the slowdown for using the threaded RTS from a factor of
7-8 down to a factor of 4-5.  I'm aiming for a factor of 2 or better...