Make cache of TriggerNames thread-safe

Changed to a thread safe container for the static cache of TriggerNames.
Merge branch 'CMSSW_7_1_X' of https://github.com/cms-sw/cmssw into makeStaticThreadSafeInEventBase
Merge pull request #2071 from Dr15Jones/makeStaticThreadSafeInEventBase

Multithreading fixes -- Make cache of TriggerNames thread-safe