jobs left incomplete when a worker is replaced

[ssube]

An image job can be left incomplete but still in the running list if:

• it is already running
• the worker hits its job limit and is replaced
• a new worker starts a new job
• one of those jobs runs out of memory

This is caused by two workers co-existing for the same device, which happens when the first worker is stuck doing a job and cannot be joined.

Cancelling the job in progress might work.

[ssube]

This is fixed for the case where a workers dies and a job is lost in the queue, but still not fully handled when a worker dies in the middle of a job and leaves it incomplete.