Various fixes for thread safety for RecoMuonL3TrackFinder

Fixes for thread safety issues with class MuonRoadTrajectoryBuilder in package RecoMuon/L3TrackFinder.
1) Both mutable data members were removed and replaced with variables on the stack, passed to member functions as arguments as needed.
2) Two non-const reverse iterators that should have been const reverse iterators were made const.
3) A reference that should have been a const reference was made const.

tested with checkdeps -a and the relval matrix.