concurrency problems with dolt_commit()

[zachmu]

Multiple concurrent sessions calling dolt_commit() can fail with the error "dataset head is not ancestor of commit".

This occurs because we determine the parent of a pending commit too early in the process, before we take a lock.

[zachmu]

Just an update: I have a prototype of a fix for this working for the original test repro at 5x the original concurrency load. We'll get it cleaned up, add some tests, and get it shipped this week.