backend: Open endpoints to peek/grab tasks with any priority
The priority notion becomes a blur. Any tasks with a non null priority is considered for reading or grabbing providing the task is ready to be scheduled.
In a future commit, this should allow to make the runner evolve to reroute tasks with priority to other queues.
Related to swh/infra/sysadm-environment#3084 (closed)
Test Plan
tox
Migrated from D5503 (view on Phabricator)