Performance improvement in quadruplet seeding (62X_SLHC)

These changes improve the CPU performance of the quadruplet seed generation used with Phase1 pixel geometry. Only the implementation of the QuadrupletSeedMerger is altered, the results should stay the same (tested with MultiTrackValidator and TrackerSeedValidator using 50 ttbar events with PU 35 in 620_SLHC1).

Forward port of PR #126.