Add explicit synchronization point.

If there's a large number of parent / child relationships, instead of having a set of all-to-all arcs (which can number into the tens of millions for many parents and children in one line), add in a explicit no-op node. This causes the number of arcs to grow linearly instead of quadratically.

HTCondor will do this automatically in future versions; this commit should be reverted once the new dagman is available.

@chad-hanna NOTE NOTE NOTE: I don't have the ability to test this; might require some adjustments before it is ready for prime-time. The basic logic is expected to be correct.