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)