Add a new WAL record type for TRUNCATE, which is only used when
wal_level >= logical.  (For physical replication, TRUNCATE is already
replicated via SMGR records.)  Add new callback for logical decoding
output plugins to receive TRUNCATE actions.

Author: Simon Riggs <simon@2ndquadrant.com>
Author: Marco Nenciarini <marco.nenciarini@2ndquadrant.it>
Author: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Reviewed-by: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>