FastTimerServiceClient: update plots after every lumisection

Adapts the interface to the 7.2.x changes to DQMEDHarvester.

This should also fix the errors reported by the CLANG builds: https://cmssdt.cern.ch/SDT/cgi-bin/buildlogs/slc6_amd64_gcc481/CMSSW_7_2_CLANG_X_2014-09-15-0200/HLTrigger/Timer .