We found several issues in the old job queue implementation. It had race
conditions, deadlocks and other deficiencies.

Short summary:
- _QueuedOpCode and _QueuedJob are now more or less data structures with a few
  utility functions. __Setup is gone.
- DiskJobStorage and JobQueue classes merged into one to reduce code complexity.
- One lock in JobQueue for almost everything. There's also a lock per opcode
  for log messages.

Reviewed-by: iustinp