Thread safety -- Make DDAxesNames thread safe

This is along the lines of what I already did in:

http://cmslxr.fnal.gov/lxr/source/CondFormats/L1TObjects/src/L1GtDefinitions.cc#256

and other places.

I've added a unit test which passes with both the new and the old code. I did not do a full rebuild. @nclopezo, can you please do it? @ianna