added index to billing for reference numbers to hopefully speed up query times.

src/sql/0-run-first/1-sanitize.sql

@@ -1,4 +1,7 @@
 DROP TABLE IF EXISTS tokens;
+DROP INDEX IF EXISTS billing_refnum ON billing;
+
+CREATE INDEX billing_refnum ON billing (refNumber);
 
 -- noinspection SqlWithoutWhere
 UPDATE users SET hash = '';