Modify 'Scheduler::getNextMount' not to get stuck on broken queues

We have had several issues related to missing shards on retrieve/archive queues:

There is a fix on the way for this particular error:

However, there was another weakness shown by these operational issues:

Selecting the next mount with 'Scheduler::getNextMount' is deterministic, given a set of inputs.
A failing queue will keep being selected, as long as it is the highest priority queue.
This can starve the system and result in a single point of failure
- Eg: A single broken archive queue will cause all archivals to get stalled

This needs to be fixed!

Some ideas for discussion:

Flag any queue that has been selected by a tape server. Do not allow it to be re-selected for N seconds.
Pseudo-randomize queue selection.
Other?