Tweak concurrency test to enqueue concurrently with workers to thrash locks a bit more.

- Since our test rig has only 20 concurrent DB connections, we only do
  concurrency=10 at a time since we insert and dequeue at same time
Refactor JQStore_Propel to use "select .. for update" instead of exclusive table locks to minimize locking issues.

- We are worried that spurious worker failures during open dequeue
  transactions combined with pg connection pooling leaving table locks
  open and preventing enqueues.

  Note that we considered advisory locks as well, but they are not more
  performant and would still be subject to the same mutex fail which
  blocks inserts.
- This also reduces the surface area of mutexes leading to increased throughput.
- Remove now-vestigial nextAlwaysWaitsForLock option.