Test suite doesn't parallelize as well as it should

[msullivan]

Some of this is probably because CREATE DATA BRANCH is serialized; we could consider loosening that restriction a bit (based on size of conn pool?) or bringing back the old testmode only database clone that uses postgres template databases.

Some might be because we have too much stuff with parallelization restrictions, and should tidy it up, and I think our algorithms about allocating those are questionable.

Some might just be inefficiency in the scheduler? I'm not sure.

[msullivan]

Getting rid of the branch semaphore takes me from 13.5m for the tests (not counting a minute for cached bootstrap) to 11.5.