Per r83812 CR, solve the categorymembers paging problem by doing separate queries...

Per r83812 CR, solve the categorymembers paging problem by doing separate queries for each value of cl_type, with some trickery to make paging work. This makes the enum->varchar schema change for cl_type unnecessary, so I'll revert that.

Code was largely copied from Tim's CR comment on r83812 but adapted to deal with the fact that we have to apply the cmcontinue-induced WHERE on cl_sortkey and cl_from only for the first query. Because ApiQueryBase doesn't let us unset or overwrite conditions like these in a nice way, I added an $extraQuery parameter to ApiQueryBase::select() that excepts additional query parameters that are only applied to that query but not stored in the object.