Add leader change event for followers

[palquel]

Fixes etcd-io/etcd#913

[philips]

This LGTM. However, does it also fix thos "empty" messages as reported in etcd-io/etcd#913?

Any objections @benbjohnson @xiangli-cmu ?

[palquel]

@philips using the etcd-io/etcd#913 example, the new logs should look like this:

[etcd] Jul 30 20:50:13.509 INFO      | app02: leader changed from 'app03' to ' '.
[etcd] Jul 30 20:50:13.509 INFO      | app02: leader changed from ' ' to 'app01'.

I think the empty is correct because it signals that the leader changed to no leader so we should expect a state change to follow (e.g. follower is promoted to candidate or new leader is assigned).