sql: is_current should only be 1 or null

(null allows multiple (ip,null) pairs, with 0 for deleted
monitors only one monitor for the ip can be deleted)

Author: abh

sql/ntppool.update

@@ -1237,6 +1237,9 @@ select id, account_id, type,
 from monitors
 where tls_name not like '%.system';
 
+#148
+update monitors set is_current=null where is_current=0;
+
 #todo