Start over with MT style tests

The existing ones had grown a bit stale as they were based entirely around DMLFunctions, relied on invasive hooks due to external API and used long sleeps to achieve ordering. DMLFunctions is on life support, the hooks weren't very flexible and long sleeps aren't amenable to FDB.

So, rip and replace. Get rid of all the existing MTs, rewrite helper code in a new style (Operator plans + CyclicBarrier) and replace the DML MT using the new helpers.

This is a nice cleanup on it's own but the real motivation is the next branch -- MTs hitting the online DDL.