Add 'racing' prerequisite for Bourreau Workers (redmine 725)

[prioux]

A new prerequisite rule could allow tasks scheduled on one Bourreau to 'race' against an identical task on another bourreau. Assume we want to process something but we can't tell which clusters are overloaded and which have some free slots. Then by lunching two identical copies of the task at the same time on two Bourreaux, each Bourreau Worker would monitor the status of the other task, and NOT launch the task if it has lost the race. The losing task would simply be cancelled and erased.