Generate more unique temp table names when the CDB_SyncTable function
is executed multiple times within the same transaction.
When executed in isolation, there will be always an implicit
surrounding transaction.
But when executed several times within the same transaction it can
give an `ERROR: relation "src_sync_718794" already exists`.
E.g:
```
BEGIN;
SELECT cartodb.CDB_SyncTable('source1', 'public', 'dest1');
SELECT cartodb.CDB_SyncTable('source12, 'public', 'dest2');
COMMIT;
```
2f8ea7e4ea