Implement priorities for redis queues

[nickpeirson]

Currently the priority argument for a job is silently ignored for redis and jobs replayed from redis are all added at the normal priority.

A possible solution would be to use hashes. If instead of calling SET key data the code called HMSET key data "data" priority "priority", and HGETALL key in the replay method, it would be possible to store and retrieve the priority from redis.

[p-alik]

Indeed it's a bug in redis/queue.cc. gearman_job_priority_t parameter is undefined.