(PDB-821) Facilitate facts subqueries against fact nodes.

This patch modifies the v4 IN and EXTRACT operators to accept multiple fields at once.
This allows the following...

    ["and" ["in", "name",
             ["extract", "name", [select-fact-nodes ["=","value",10]]]]
           ["in", "certname",
             ["extract", "certname", [select-fact-nodes ["=", "value", 10]]]]]

to be re-written as

    ["in", ["name","certname"],
      ["extract", ["name", "certname"], ["select-fact-nodes", ["=", "value", 10]]]]

As written, this will affect in and extract for all v4 endpoints, while throwing an
exception on v2/v3 to explain that vector fields are not allowed.
Merge pull request #1053 from wkalt/ticket/master/pdb-821-fact-nodes-subquery

(PDB-821) Facilitate facts subqueries against fact nodes.