Refactored search_special_fields() into search_unit_by_times_cited(). Removed unnecessary list/set conversions.  Plugged the function into search_unit() so that low-level API can use "cited:" index too.  Fixed record universe detection so that "cited:" works in restricted collections too.
Refactored search_special_fields() into search_unit_by_times_cited().
Removed unnecessary list/set conversions.  Plugged the function into
search_unit() so that low-level API can use "cited:" index too.  Fixed
record universe detection so that "cited:" works in restricted
collections too.

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio