#968830 by q0rban: Abstract direct db function calls in query plugins to facilitate plugins that just swap out the actual db interaction.

#648438 by dww: In rare circumstances, unnecessary term_node tables could be joined in when adding term_data relationships.

#619812 by chx: Break up the query() method on the query plugin to have field compilation be separate.

#512650 by sdboyer: Multiple many to one arguments on the same relationship were not relationship safe.

Fix accidentally reverted changes. Plus get rid of an ugly piece of hardcoding around the signature.