Split `mnt` job into more granular items

[marcopeg]

Right now, by running the job named mnt from the fetchq.jobs queue, the following functions are called on a single queue:

• fetchq.mnt_mark_dead()
• fetchq.mnt_reschedule_orphans()
• fetchq.mnt_make_pending()

By running so much stuff all together, we increase the possibility for introducing a dead lock on the table as we are updating multiple rows at the time, and also dealing with stats updates.

We can introduce specific lines in the jobs table to run each of them in isolation, reducing the single maintenance time hit, reducing the probability for dead locks