Previously, jobs were executed in FIFO order on MongoDB, and LIFO on
Postgres, with no way to configure this behavior.
This PR makes FIFO the default on both MongoDB and Postgres and
introduces the following new options to configure the processing order
globally or on a queue-by-queue basis:
- a `processingOrder` property to the jobs config
- a `processingOrder` argument to `payload.jobs.run()` to override
what's set in the jobs config
It also adds a new `sequential` option to `payload.jobs.run()`, which
can be useful for debugging.
