Output file sequencer adaptation for AthenMT

Changes to AthenaOutputStream and OutputStreamSequencerSvc to allow output file sequences (automatic splitting of the output file into smaller chunks with sequence number added to the filename) to work in AthenaMT. Added protections (mutexes) to allow writing of different chunks concurrently. Small fix to ensure EventContext is present during EndProcessing incident. Output sequences in AthenaMP should work as before (including joboptions configuration)