Cluster: decrease ping/pong traffic by trusting other nodes reports.

Cluster of bigger sizes tend to have a lot of traffic in the cluster bus
just for failure detection: a node will try to get a ping reply from
another node no longer than when the half the node timeout would elapsed,
in order to avoid a false positive.

However this means that if we have N nodes and the node timeout is set
to, for instance M seconds, we'll have to ping N nodes every M/2
seconds. This N*M/2 pings will receive the same number of pongs, so
a total of N*M packets per node. However given that we have a total of N
nodes doing this, the total number of messages will be N*N*M.

In a 100 nodes cluster with a timeout of 60 seconds, this translates
to a total of 100*100*30 packets per second, summing all the packets
exchanged by all the nodes.

This is, as you can guess, a lot... So this patch changes the
implementation in a very simple way in order to trust the reports of
other nodes: if a node A reports a node B as alive at least up to
a given time, we update our view accordingly.

The problem with this approach is that it could result into a subset of
nodes being able to reach a given node X, and preventing others from
detecting that is actually not reachable from the majority of nodes.
So the above algorithm is refined by trusting other nodes only if we do
not have currently a ping pending for the node X, and if there are no
failure reports for that node.

Since each node, anyway, pings 10 other nodes every second (one node
every 100 milliseconds), anyway eventually even trusting the other nodes
reports, we will detect if a given node is down from our POV.

Now to understand the number of packets that the cluster would exchange
for failure detection with the patch, we can start considering the
random PINGs that the cluster sent anyway as base line:
Each node sends 10 packets per second, so the total traffic if no
additioal packets would be sent, including PONG packets, would be:

    Total messages per second = N*10*2

However by trusting other nodes gossip sections will not AWALYS prevent
pinging nodes for the "half timeout reached" rule all the times. The
math involved in computing the actual rate as N and M change is quite
complex and depends also on another parameter, which is the number of
entries in the gossip section of PING and PONG packets. However it is
possible to compare what happens in cluster of different sizes
experimentally. After applying this patch a very important reduction in
the number of packets exchanged is trivial to observe, without apparent
impacts on the failure detection performances.

Actual numbers with different cluster sizes should be published in the
Reids Cluster documentation in the future.

Related to #3929.

src/cluster.c

@@ -1354,6 +1354,19 @@ void clusterProcessGossipSection(clusterMsg *hdr, clusterLink *link) {
                 }
             }
 
+            /* If from our POV the node is up (no failure flags are set),
+             * we have no pending ping for the node, nor we have failure
+             * reports for this node, update the last pong time with the
+             * one we see from the other nodes. */
+            if (!(flags & (CLUSTER_NODE_FAIL|CLUSTER_NODE_PFAIL)) &&
+                node->ping_sent == 0 &&
+                clusterNodeFailureReportsCount(node) == 0)
+            {
+                uint32_t pongtime = ntohl(g->pong_received);
+                if (pongtime > node->pong_received)
+                    node->pong_received = pongtime;
+            }
+
             /* If we already know this node, but it is not reachable, and
              * we see a different address in the gossip section of a node that
              * can talk with this other node, update the address, disconnect