Make card_number more central for CORE API -> Rattail importers

let's track that as (effectively) `Customer.corepos_card_number` and
use that when possible for importer key

rattail_corepos/corepos/util.py

@@ -46,7 +46,9 @@ def get_core_members(config, api, progress=None):
 
     # first we fetch all customer records from CORE DB
     with app.short_session(factory=CoreSession) as s:
-        db_customers = s.query(corepos.CustData).all()
+        db_customers = s.query(corepos.CustData)\
+                        .order_by(corepos.CustData.card_number)\
+                        .all()
         s.expunge_all()
 
     # now we must fetch each customer account individually from API