CRM-17075 switch membership_type_id to use entity reference

CRM-17075 switch membership_type_id to use entity reference

This commit also adds generic handling for changing select fields to
entity reference fields with tests to ensure previous smart group
behaviour is respected by handling within the convertFormValues function.

This reverts advanced search domain filtering to ?4.4? 4.5? behaviour which does not assume only that domain's memberships should be searched on.

It make be that domain filtering + allowing an override makes most sense. I did think the pseudoconstant & getoptions use the same
mechanism, but it seems they don't. Really they should both allow the same behaviour & override