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.
Cluster: don't forget yourself with CLUSTER FORGET.
Cluster: redis-trib help output improved.

Show options if any. Clarify that for some command any node address is
ok.
Cluster: don't let a node forget its own master.

redis-trib should make sure to reconfigure slaves of a node to remove
from the cluster to replicate with other nodes before sending CLUSTER
FORGET.
Cluster: redis-trib delnode implementation.
Cluster: fix redis-trib help subcommand.
Cluster: redis-trib addnode is now able to add replicas.
Cluster: redis-trib shows what a slave replicates + fixes.

Also the :replicates info field in the node object is now correctly
populated. This also fixes the :replicas field computation.
Cluster: redis-trib help output format modified.
Cluster: redis-trib shows number of replicas of masters.
Set server.repl_down_since to 0 when changing master.

When an instance is potentially set to replicate with another master, it
is conceptually disconnected forever, since we have no old copy of the
dataset for this master in memory.
Cluster: allow CLUSTER REPLICATE to switch master.

The code was doing checks for slaves that should be done only when the
instance is currently a master. Switching a slave from a master to
another one should just work.
Cluster: fix error reporting when slaveof is found in config.