Query-retrieve operations need their own queue

[Ed McDonagh]

The C-Find and C-Move operations can take a long while, and can block the resulting extractions from being processed till the move is complete.

Propose a new qr queue to join the stores and default queues.

Also currently a move called from a query is not sent to Celery, but rather continues in the parent's task. Propose breaking this into a new task.

[Ed McDonagh]

Implemented the changes proposed in ref #326. Need to change documentation to reflect this before closing task.

→ <<cset 4cff4df16e85>>

[Ed McDonagh]

Removed references to specific qr queue again after changing prefetch to 1, as separate qr queue should no longer be necessary. Refs #326

→ <<cset 1164cf67f034>>

[Ed McDonagh]

Resolved back in December. Would only useful if several queries are taking place at once and took all available workers.