Lower eunit concurrency to 100 from 1500

OS X 10.9 has annoyingly low open file limits. The
old eunit tests were running 1500 connections in parallel,
which new OS X doesn't like.

Now we limit the tests to running 100 in parallel (instead of
the old running all 1500 at once).  Limiting to 100 seems to work
fine with default OS X 10.9 ulimits.