Improve leader election convergence

Improve leader election convergence

When 2 nodes startup with the first node's log behind the second node's,
it usually takes several election rounds to converge - I've seen
anywhere from 40 s to 3 min, depending on timing. What happens is that
the first node goes to Candidate first but it's RequestVote is rejected
by the seconds node. Shortly after the seconds node goes to Candidate -
the term is higher than the first which causes the first node to go back
to Follower. However it doesn't respond to the RequestVote. Then the
first node goes to Candidate and the cycle repeats. Eventually, due to
the election variance, the seconds node times out first and the second
node process the RequestVote and grants it. But it can take more than 10
cycles.

We can improve the convergence by allowing a Candidate to process and
respond to RequestVote when the sender's term is greater. It still
transitions to Follower as per the raft rules. The raft paper does not
say whether or not a Candidate can/should process a RequestVote in this
case but it seems to make sense. With this change, the first RequestVote
sent by the second node is granted and it converges quickly.

Change-Id: If9416ddf7bf0dfc1220a169be4174f440626a0dd
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>