Feature: ability to set a priority to certain workflows

[keevan]

There might be a newer workflow created, that should be run as top priority. But it seems currently it's running in the order it was created at, and in the mix is the process for reprocessing failed steps.

Similar to recent works of CRON QoS, maybe we can implement the same behaviour, or more simply, have a customisable priority setting for each workflow, which might also take into account if the step had previously failed to run or not, allowing it to process steps that don't take long / have a higher priority / previously passed first.

The failure check might be trickier than it sounds though since it might fail a condition as expected, or it might be exploding somewhere instead.