RPC DQM - Moving to DQMEDHarvesters

RPC clients have been migrated to the DQMEDHarvester interface as described in: https://twiki.cern.ch/twiki/bin/viewauth/CMS/ThreadedDQM.
Plus cleanups and bug fixes.

This PR supersedes my previous one (https://github.com/cms-sw/cmssw/pull/5410)


this one also should be kept I guess....
see:
https://cmssdt.cern.ch/SDT/cgi-bin/buildlogs/slc6_amd64_gcc481/CMSSW_7_3_X_2014-10-23-1400/pyRelValMatrixLogs/run/25.0_TTbar+TTbarINPUT+DIGI+RECO+HARVEST+ALCATT/step4_TTbar+TTbarINPUT+DIGI+RECO+HARVEST+ALCATT.log

I will fix it...