Fix incorrect cond construction in _minimal_valueset_satisfying_constraint

The function in question introduced in d681f1bb (which builds upon work in
8e40a627) correctly uses _extract_fixed_condition_columns, but then fails to
account for all literals having their leading { '=' => ... } being stripped.

While it is unfortunate that I need to reexamine values through the function
again, there doesn't seem to be a clean solution to this: all other invocants
of _extract_fixed_condition_columns expect a valuebag, not a condition.

Therefore just add things back and move on
Ensure ::Schema::Versioned connects only once by reusing the main connection