space_upgrade: respect min_field_count of both old and new formats

When upgrading a space, attribute `has_optional_parts` of indexes can be
changed. So in order to correctly index both old and new tuples we should
set new min_field_count value to the minimal min_field_count of old and
new formats. Actual value will be set when space upgrade completes.

Part of tarantool/tarantool-ee#698
Part of tarantool/tarantool-ee#750

NO_TEST=in ee
NO_CHANGELOG=in ee
NO_DOC=bugfix

(cherry picked from commit c449ada4)