delete certs that have long expired (6 months)

fixes #783

src/blobs.js

@@ -5,6 +5,7 @@
 exports = module.exports = {
     get,
     set,
+    del,
 
     initSecrets,
 
@@ -45,6 +46,10 @@ async function set(id, value) {
     await database.query('INSERT INTO blobs (id, value) VALUES (?, ?) ON DUPLICATE KEY UPDATE value=VALUES(value)', [ id, value ]);
 }
 
+async function del(id) {
+    await database.query('DELETE FROM blobs WHERE id=?', [ id ]);
+}
+
 async function clear() {
     await database.query('DELETE FROM blobs');
 }