Better queue balancing

For the moment the background job queue is quite straightforward and each workers take the oldest not taken task (exactly they can the lowest between the insertion datetime and the number of error, so a failed task will be retried only when the queue is empty).

This leads to a big sender blocking the queue for all the others, and it will be nice to find a better balancing.

@cquest has a prototype with a random selection of an account (this will lead to better balancing between users), but I'm not a fan of the randomness here.