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.