feat: Prune all orphan branches (#461)

Schedule logic has been refactored to enable the pruning of all orphan branches. 
Now, schedules aren't checked at package-time, instead package upgrades are queued regardless of schedule. 
At branch time, it is checked whether the *branch* is scheduled and then the branch is not created/updated if it's off-schedule.
This enables the repository worker to know all possible branches and hence be able to determine which remaining branches in the repository are "orphans" to be deleted.

Closes #428, Closes #426