Add sequence support for externallinks table

Database field el_id was added with
Ib6b0fc3736d173fa4ba7b786ecfc710b2f4711bb, but due to autoincrement in
mysql no sequence support was added.

For mysql this is a nop, it adds a field to the insert but that is null
and mysql autoincrement will fill it.
INSERT /* LinksUpdate::incrTableUpdate 127.0.0.1 */ IGNORE
  INTO `externallinks` (el_id,el_from,el_to,el_index)
VALUES
(NULL,'1610','http://de.wikipedia.org/wiki/Hauptseite','http://org.wikipedia.de./wiki/Hauptseite')

At least Oracle will need it

Bug: 58435
Change-Id: I3f68ea8d99c5450c0c66feeb1469ef0ee6cc4eab