In working on the next iteration of the lock manager to provide
object-level locking, I realized:

- It was saner let all waiting try to get locks when locks are
  released, at least in the more complicated logic to follow.

- We really do almost certianly want a multi-threaded server, even if
  it doesn't run faster (still an open question), because otherwise,
  big commits will completely block loads.

- We don't really want to hold the lock-manager lock while calling the
  callback.  Again, this really only matters if we have a
  multi-threaded server, but it also feels like a matter of hygiene :)

I decided to rework this branch:

- Don't hold lock-manager internal lock when calling the callnack.

- When releasing the lock, use call_soon_threadsafe to let all waiting
  have a chance to get the lock.

- A little bit of factoring to DRY. (This factoring will be much more
  useful in the follow-on branch.

This rework restores the workability of the thread-per-client model.