See https://www.postgresql.org/docs/9.6/static/runtime-config-replication.html#GUC-HOT-STANDBY-FEEDBACK for info about the option.
Enabling this cuts down the number of query conflicts on secondaries to 0:
Here the option was manually enabling a bit before 17:00. After enabling this I did not notice any change in table bloat. For the sake of safety the option is still disabled by default
@marin I added the CHANGELOG entry to 9.0.1. Is this something we're OK with releasing in a patch release, or do we want to wait for 9.1 (or somehow ninja it into 9.0)?