Do not update term from unreachable members

Do not update term from unreachable members

In case of a one way connection problem, a follower that cannot
receive messages but can send them, it will change into Candidate
and continuosly bump its term while it cannot receive any
RequestVoteReplies.

Since the messages it sends can actually get through, this means
that the healthy part of the cluster will have its leader dropped
every 2 election timeouts.

Change this up so we only update term when receiving a RequestVote
from a reachable node.

Change-Id: Ia8a0b59cfb2d0cd68096172a1d2d47f68e7ed473
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>