Dqm multithread

First chunk of the new MultiThread DQM Framework.
Still somewhat a work in progress.
Could profit from hints by Chris on reducing the number of internal indices used to properly address ME: yet internal detail that does not affect the huge migration from EDAnalyzer to DQMEDAnalyzer ahead of us.

The changes have been implemented in a backward compatible way, i.e. DQM+Validation should keep on working for both old(EDAnalyzers) and new(DQMEDAnalyzers) code.