jqueue: Return jobs to queue when shutting down

When a job is still waiting for locks and the queue is shutting down,
they should be returned and not actually start processing. Until now
jobs which transitioned from “queued” to “waiting” were already
considered to be running as far as the shutdown code was concerned.

This fixes issue 296.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>