Avoid calling getProcessParameterSet

Now get the top level ParameterSet from the edm::Event
Make getProcessParameterSet* thread-safe

Access to the top level process ParameterSetID is now protected by a mutex.
NOTE: We should eventually have a campaign to remove the use of this function since it is ambiguous when used with a job that contains a SubProcess.
Merge pull request #6384 from Dr15Jones/makeProcessParameterSetThreadSafe

Make getting Process ParameterSet thread-safe
Merge remote branch 'origin/CMSSW_7_3_X' into CMSSW_7_0_X