FastTimerService: fix DQM directory structure

FastTimerService:
  - move per-path plots under the per-process directories

FastTimerServiceClient:
  - adapt the harvesting step to the new plot directory structure
  - cleanup
Merge pull request #3089 from fwyzard/FastTimerService_fix_directory_structure

HLTrigger/Timer -- FastTimerService: fix DQM directory structure