Simplifies the `ready_prs` query a bit and allows it to be converted
to an ORM search, by moving the priority check outside. This also
allows the caller to not need to post-process the records list
anywhere near the previous state of affairs.
`ready_prs` now returns *either* the "alone" batches, or the non-alone
batches, rather than mixing both into a single sequence. This requires
correctly applying the search filters to not retrieve priority of
batches in error or targeting other branches.
