FastTimerServiceClient: update plots after every lumisection

Adapt the DQM clients to the 7.2.x DQMEDHarvesting interface, which is necessary to properly update histograms online, and in general when running the DQM harvesting with the "Online" convention.

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 . 
Automatically ported from CMSSW_7_2_X #5305