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.
redis-cli: --latency-history mode implemented.
redis-cli: raise error on bad command line switch.

Previously redis-cli never tried to raise an error when an unrecognized
switch was encountered, as everything after the initial options is to be
transmitted to the server.

However this is too liberal, as there are no commands starting with "-".
So the new behavior is to produce an error if there is an unrecognized
switch starting with "-". This should not break past redis-cli usages
but should prevent broken options to be silently discarded.

As far the first token not starting with "-" is encountered, all the
rest is considered to be part of the command, so you cna still use
strings starting with "-" as values, like in:

    redis-cli --port 6380 set foo --my-value