Cluster: use long long for timestamps in clusterGenNodesDescription().

Ping sent and pong received fields need to be casted to long long to be
printed correctly into 32 bit systems.
Fixed clearNodeFailureIfNeeded() time type to mstime_t.

This prevented 32bit cluster instances from clearing the FAIL flag when
needed.
Cluster: use an hardcoded 60 sec timeout in redis-trib connections.

Later this should be configurable from the command line but at least now
we use something more appropriate for our use case compared to the
redis-rb default timeout.
Cluster: use proper type mstime_t for ping delay var.
Cluster: initialize todo_before_sleep flags to 0.
Cluster: check link is valid before sending UPDATE.
Cluster: set n->slaves to NULL in clusterNodeResetSlaves().

The value was otherwise undefined, so next time the node was promoted
again from slave to master, adding a slave to the list of slaves
would likely crash the server or result into undefined behavior.