Since `synchronous_mode` was introduced to Patroni, the plain Postgres synchronous replication has been no longer working.
The issue occurs because `process_sync_replication` always resets the value of `synchronous_standby_names` in Postgres when `synchronous_mode` is disabled in Patroni.
This commit fixes that issue by setting the value of `synchronous_standby_names` as configured by the user, if that is the case, when `synchronous_mode` is disabled.
Closes#3093
References: PAT-254.