Optimization of PixelHitMatcher

Factor of 3.5 speed up in ecalDrivenElectronSeeds producer.
4x speed up in compatibleSeeds() function in PixelHitMatcher class.
Mostly this comes from using hash tables, some speedup from not using GlobalPoint::phi() all the time and lazy returns.

Minor knock-on timing improvements in conversion tracking.

No regressions expected.
In local testing of single electron gun and ttbar 25ns, no regressions observed.

@VinInn