Overhaul the Event system implementation to follow the structure of EventSetup

[makortel]

The rigidity of the Event system is starting show problems for the evolution of the framework. Some symptoms

• ExternalWork ability requires the developer to explicitly store the data to be passed from acquire() to produce() https://github.com/cms-sw/cmssw/issues/24186
• The mapping of BranchDescription to a corresponding ProductResolver type is complicated https://github.com/cms-sw/cmssw/blob/e5ac8e997d5565e91eea5eaa91884a2b17a0fde4/FWCore/Framework/src/Principal.cc#L150-L216
  • SwitchProducer required two concrete ProductResolvers and an intermediate base class
  • Transformer required a new ProductResolver type

The EventSetup system is much more flexible in this regard. At some point it would become cheaper to revamp the Event system to use the similar (same?) internals as the EventSetup system.

[makortel]

This ability would be useful for multiprocessing across nodes as well (see https://indico.cern.ch/event/1480583/#19-update-on-cmssw-clientserve for what motivated this conclusion).

The idea would be to make the ProductResolvers to be the communication points across processes rather than the modules.