Cluster: fix logic to detect we are among a minority.

In the cluster evaluation function we are supposed to set the cluster
state as "fail" if we are among a minority, however the code was not
detecting to be into a minority partition if exactly half the masters
were reachable, which is a minority.
Cluster: process gossip section only for known nodes.

With the exception of nodes sending MEET packets: we have to trust them
since they can send us MEET packets only when the cluster is initially
created or because sysadmin manual action.
Redis 2.9.101 (3.0.0 Release Candidate 1).