Provide a way to transfer leadership via API or CLI

[hanshasselberg]

This would be great for example when upgrading a consul cluster. Before restarting the leader, it could transfer leadership which is faster because it doesn't need to wait for the electionTimeout to fire. Requires hashicorp/raft#306.

[jsosulska]

@nicholasjackson mentioned is that there should be a way to ensure that when this is called, the previous leader doesn't get reelected.

One recommendation was to grant a temporary non-voting status to the leader, then bringing up a new instance afterwards

[jkirschner-hashicorp]

Note: it looks like Vault implemented this in hashicorp/vault#1146, which was done prior to the (also now merged) hashicorp/raft#306. It seems like the Vault approach restarts the leader without first transferring leadership.