When the certificate of leader is rotated, it will certainly be
disconnected after reconfiguring communication. Instead of waiting
for ElectionTimeout and elect new leader, the old leader should be
more cooporative and transfer its leadership to others.

Note that proposals sent during this transition will be automatically
dropped by etcd/raft, however transition should be fairly short.

Change-Id: Iabd005d00864afe09b4738f1ed36b939b1d83eed
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>