raft.c: Set candidate_retrying if no leader elected since last election.

raft.c: Set candidate_retrying if no leader elected since last election.

candiate_retrying is used to determine if the current node is disconnected
from the cluster when the node is in candiate role. However, a node
can flap between candidate and follower role before a leader is elected
when majority of the cluster is down, so is_connected() will flap, too, which
confuses clients.

This patch avoids the flapping with the help of a new member had_leader,
so that if no leader was elected since last election, we know we are
still retrying, and keep as disconnected from the cluster.

Signed-off-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>