When multiple instance of celery-monitor are running, we could have an
integrity errors raised if two instance are working on the same task_id or
temporary (network, host) failures. In this case we want to retry handling the
task_id later.

Put pending processed task_id in a "pending queue" and each minutes and if the
monitor queue is empty, requeue pending items.

This change require to handle the "timeout" parameter of loop (only used in
tests) in a different way to ensure not blocking forever in redis "brpoplpush".