Added reset method to AtomicPtrCache

After using AtomicPtrCache in real code, we found that a dedicated reset() method makes the code cleaner. Previously one had to create a temporary empty AtomicPtrCache and assign it to the old variable in order to reset.
Merge pull request #1371 from Dr15Jones/addResetToAtomicPtrCache

Multithreaded framework -- Added reset method to AtomicPtrCache
Merge remote-tracking branch 'origin/CMSSW_7_0_X' into CMSSW_7_0_X