Make rio.c comment 80-columns friendly.
Cluster: turn hardcoded node timeout multiplicators into defines.

Most Redis Cluster time limits are expressed in terms of the configured
node timeout. Turn them into defines.
Cluster: node timeout is now configurable.
Cluster: CONFIG GET cluster-node-timeout.
Cluster: move REDIS_CLUSTER_FAILOVER_DELAY near other timing defines.
Test: remove useless statements and comments from test default config.
Cluster: properly send ping to nodes not pinged foro too much time.

In commit d728ec6 it was introduced the concept of sending a ping to
every node not receiving a ping since node_timeout/2 seconds.
However the code was located in a place that was not executed because of
a previous conditional causing the loop to re-iterate.

This caused false positives in nodes availability detection.

The current code is still not perfect as a node may be detected to be in
PFAIL state even if it does not reply for just node_timeout/2 seconds
that is not correct. There is a plan to improve this code ASAP.
Cluster: configdigest field no longer used. Removed.
Cluster: use server.cluster_node_timeout directly.

We used to copy this value into the server.cluster structure, however this
was not necessary.

The reason why we don't directly use server.cluster->node_timeout is
that things that can be configured via redis.conf need to be directly
available in the server structure as server.cluster is allocated later
only if needed in order to reduce the memory footprint of non-cluster
instances.
Cluster: CONFIG SET cluster-node-timeout.
Cluster: reconfigure additonal slaves on failover.