Made static const for BeamSpotTransientTrackingRecHit

By moving the initialization to library load time, the static used
by BeamSpotTransientTrackingRecHit can be made const. This avoids
thread-safety issues.
Merge pull request #3580 from Dr15Jones/makeStaticConstForBeamSpotTransientTrackingRecHit

Reco -- Made static const for BeamSpotTransientTrackingRecHit