Make 2i pagination sort optional

This addresses the performance penalty from the new default of sorting
2i results since pagination was added. When paginating, (max_results
set) sorting is always enabled. But when not, it will only be used if
configured in app.config or specified in the client request.
Continuation implies pagination sort

Even if max results is not passed, using a continuation implies an
expectation of results ordered in the standard paginating way.