Cluster: time switched from seconds to milliseconds.

All the internal state of cluster involving time is now using mstime_t
and mstime() in order to use milliseconds resolution.

Also the clusterCron() function is called with a 10 hz frequency instead
of 1 hz.

The cluster node_timeout must be also configured in milliseconds by the
user in redis.conf.
Cluster: example redis.conf updated from sec to ms for cluster-node-timeout.
Cluster: clusterCron() freq is now 10h. Still ping 1 node every sec.

After the change in clusterCron() frequency of call, we still want to
ping just one random node every second.
Cluster: data_age conversion to milliseconds fixed.