Cluster: atomic update of nodes.conf file.

The way the file was generated was unsafe and leaded to nodes.conf file
corruption (zero length file) on server stop/crash during the creation
of the file.

The previous file update method was as simple as open with O_TRUNC
followed by the write call. While the write call was a single one with
the full payload, ensuring no half-written files for POSIX semantics,
stopping the server just after the open call resulted into a zero-length
file (all the nodes information lost!).
Cluster: ignore empty lines in nodes.conf.

Even without the user messing manually with the file, it is still
possible to have blank lines (just a single "\n" per line) because of
how the nodes.conf update/write process works.
Cluster: range checking in getSlotOrReply() fixed.

See issue #1426 on Github.
Cluster: periodically call clusterUpdateState() when cluster is down.

Usually we update the cluster state (to understand if we should accept
queries or reply with an error) only when there is a change in the state
of the nodes. However for the "delayed rejoin" feature to work, that is,
for a master to wait some time before accepting queries again after it
rejoins the majority, we need to periodically update the last time when
the node was partitioned away from the majority.

With this commit if the cluster is down we update the state ten times
per second.
Cluster: set a minimum rejoin delay if node_timeout is too small.

The rejoin delay usually is the node timeout. However if the node
timeout is too small, we set it to 500 milliseconds, that is a value
chosen to be greater than most setups RTT / instances latency figures
so that likely communication with other nodes happen before rejoining.
Cluster: clusterBlacklistExists() requires blacklist cleanup before lookup.
Cluster: clusterBlacklistAddNode() key lookup fixed.

We can't lookup by node->name that's not an SDS string but a plain C
array in the node structure.
Cluster: fix clusterBlacklistAddNode() by setting right expire time.

The hash table value should be set to now + 60 seconds otherwise it
expires immediately.
Cluster: use the node blacklist in CLUSTER FORGET.

CLUSTER FORGET is not useful if we can't remove a node from all the
nodes of our cluster because of the Gossip protocol that keeps adding
a given node to nodes where we already tried to remove it.

So now CLUSTER FORGET implements a nodes blacklist that is set and
checked by the Gossip section processing function. This way before a
node is re-added at least 60 seconds must elapse since the FORGET
execution.

This means that redis-trib has some time to remove a node from a whole
cluster. It is possible that in the future it will be uesful to raise
the 60 sec figure to something bigger.