feat(standby): notice standby on leader change event

This is the most naive way to update leader address info for
standby mode instances.
Add 'admin/config/standby' API to update standby info.
Add tests about killing leader in a cluster that has standby-mode
instances.
chore(peer_server): rename 'admin/config' to 'admin/config/cluster'

To match with 'admin/config/standby'