Work Service can double-dequeue!

[analogrelay]

This is a Bad Thing(TM). We have a race in the DequeueInvocation SPROC where it can end up modifying the same invocation twice. We were being too aggressive about optimizing locks and ended up under-locking. Since this is such a fast and relatively rare operation on a low-traffic DB, let's just lock the whole darn thing.

Test Notes: This is basically a heisenbug, meaning it's really hard to repro, but it always seems to show up with the copy database job, so as long as the backups are being created and not flooding the server with "copytemp_" databases, we're good.