If tick interval and election timeout are fairly large (500ms and
10 by default), it would take a newly created channel 5-10 seconds
to elect a leader and start serving requests. If there's a split vote,
leaderless period can be prolonged further.

This CR starts proactive campaign instead of passively wait for timeout.

Change-Id: Ife0e5b8bd7e2b52c3fde8ba887ffe99a5e1ff0ab
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>