Diffing_Engine: formalise distinction between Diffing and Hash.

[alelom]

Description:

• Current HashFragment that stores both current/old hash must be renamed to RevisionFragment. It was designed to be used with a "diffing-revision workflow" anyway (so you can track objects back in time and tell who is new/modified/old).
• Introduce new HashFragment that only stores the current hash. This is to be used for "diffing-current state workflow".
• Review the IDiffing() method:
  • to make sure that by default computes HashFragment for objects and uses that (do a "diffing-current state");
  • if instead a RevisionFragment is found on the objects, do a "diffing-revision";
  • make sure that in the "diffing-current state", the property-by-property diffing is off by default (priority must be speed).
  • make sure that in the "diffing-revision", the property-by-property diffing is true by default (priority must be complete comparison).

Also consider:

• Introduce new HashConfig? To be used as input to the BH.Engine.Base.Query.Hash() method instead of the long list of inputs.
• Allow conversion/mapping between DiffConfig-HashConfig?
• Allow conversion between RevisionFragment and HashFragment?

@rolyhudson @al-fisher