Proposal: Revise TAXII Query

[MarkDavidson]

I haven't fleshed out a full set of topics, but there are some hurdles I've identified while implementing TAXII Default Query in django-taxii-services.

• Identifying which properties (Content Bindings, Capability Modules, Targeting Expression IDs, Targeting Expressions) of TAXII that are used in any given TAXII Default Query requires quite a bit of bookkeeping. Matching the properties of a given TAXII Default Query against available query handlers is even more problematic. It would be nice if all properties were stated up front in the query.
  Example: https://github.com/TAXIIProject/django-taxii-services/blob/master/taxii_services/models.py#L1445
• Implementers seem to have a hard time mapping targeting expressions to their data store. This is far an away the most frequent question I receive regarding query. Either the concept is not straightforward enough or the explanation of the concept can be improved. Either way, it should be revisited.