This allows to cancel commands run over replication connections. While
it might have some use before v10, it has become important now that
normal SQL commands are allowed in database connected walsender
connections.

Author: Petr Jelinek
Reviewed-By: Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/7966f454-7cd7-2b0c-8b70-cdca9d5a8c97@2ndquadrant.com