Optimized allocation of TES slot in HiveWhiteBoard

Sebastien Ponce requested to merge lhcb/Gaudi:sponce_HiveWBOpt into master

This was doing a linear search on the slots, taking a lock on each of them to check their availability. Not only the algorithm was O(n) while it could be O(1) but the locking was not scaling when many threads were at work. Putting a fifo queue with its own small lock solves the issue.

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