The apply pending actions can double-post when having concurrent requests

[suhaibmujahid]

Note

This did not happen yet, so no examples, but in theory it is a bug that needs to be fixed.

The apply_all_pending method reads a run's not-yet-applied rows, then dispatches and commits each one, with no lock. Two concurrent calls both see the same pending rows and both dispatch them, so the same Bugzilla comment can be posted twice.