MDL-60174 core_dml: fix miscellaneous incorrect recordset usage

The new recordset support for Postgres requires transactions and
will cause errors if recordsets are not closed correctly. This
commit fixes problems that were identified during unit tests, and
via some basic code analysis, across all core code. Most of these
are incorrect usage of recordset (forgetting to close them).

admin/cli/fix_deleted_users.php

@@ -73,6 +73,7 @@ foreach ($rs as $user) {
     echo "Redeleting user $user->id: $user->username ($user->email)\n";
     delete_user($user);
 }
+$rs->close();
 
 cli_heading('Deleting all leftovers');