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;
```
With javitonino's help, greatly reduce the processing time by using
EXCEPT instead of NOT IN, which causes it to use a `HashSetOp Except`
plan on the subqueries rather than a `Seq Scan` on `Materialize`'d
subtables.
It assumes there's a cartodb_id column in both source and target. It
does not perform unnecessary actions. It respects augmented columns in
target table, if they exist. It is meant to be efficient.
