Optimize the heap usage in HistogramCombineGreedy.
Optimize the heap usage in HistogramCombineGreedy.

The previous priority system used a heap which was too heavy to
maintain (what was gained from insertions / deletions was lost
due to a linear that still happened on the heap for invalidation).
The new structure is a priority queue where only the head is
ordered.

Change-Id: Id13f8694885a934fe2b2f115f8f84ada061b9016

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