Fixed thread-safety issues with PixelToFEDAssociateFromAscii

Changed the class statics to be member data. This avoids potential
thread-safety issues. The additional minor changes were to support
this change or to make mainentance easier. The exact functionality
of this class was unchanged.
Merge pull request #2775 from Dr15Jones/makePixelToFEDAssociationFromAsciiThreadSafe

CalibTracker/SiPixelConnectivity -- Fixed thread-safety issues with PixelToFEDAssociateFromAscii