Print WAL logical message contents in pg_waldump

This helps debuggability when looking at WAL streams containing logical
messages.

Author: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CAExHW5sWx49rKmXbg5H1Xc1t+nRv9PaYKQmgw82HPt6vWDVmDg@mail.gmail.com

src/backend/replication/logical/message.c

@@ -59,6 +59,7 @@ LogLogicalMessage(const char *prefix, const char *message, size_t size,
 
 	xlrec.dbId = MyDatabaseId;
 	xlrec.transactional = transactional;
+	/* trailing zero is critical; see logicalmsg_desc */
 	xlrec.prefix_size = strlen(prefix) + 1;
 	xlrec.message_size = size;