Replica crashes when decreasing `max_connections`

The `fly pg config update` tool will currently perform a rolling-restart in order to apply configuration changes that cannot be applied at runtime. As of right now, all restarts will start with the replicas and end with the primary.

This process does not work in some cases when adjusting configuration values down.

As an example, a replica will not boot if its `max-connections` value is lower than the value specified by the primary.