Add config option to default an alternate query processor

[bruth]

This brings the new query processor support full circle. This is still limited to using a single query processor for a session, but there is not a clear use case for using multiple query processors at the same time in Cilantro.

[gracebrownecodes]

PEDSnet has a definite use case for using multiple query processors in the same session, which is to switch between whole-dataset stats and authorization-limited-dataset stats on the query page. Documented here (private to pedsnet members).