Foremen send info to master

Three main changes:

1) Stats of the queue are now kept on a work_queue_stats structure.
2) Workers can send "info field value" to the master. Such messages can be ignored by the master.
3) work_queue_get_stats_hierarchy  is similar to work_queue_get_stats. The later has only statistics of workers connected to the master, and the former accumulates statistics from the master and all the foremen.