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