A client can enter a special cluster read-only mode using the READONLY
command: if the client read from a slave instance after this command,
for slots that are actually served by the instance's master, the queries
will be processed without redirection, allowing clients to read from
slaves (but without any kind fo read-after-write guarantee).

The READWRITE command can be used in order to exit the readonly state.

long was used instead of long long in order to handle a 64 bit
resolution millisecond timestamp.

This fixes issue #1483.

The function actually needs to be split into sub-functions at some
point in the future.

When the configured node timeout is very small, the data validity time
(maximum data age for a slave to try a failover) is too little (ten
times the configured node timeout) when the replication link with the
master is mostly idle. In this case we'll receive some data from the
master only every server.repl_ping_slave_period to refresh the last
interaction with the master.

This commit adds to the max data validity time the slave ping period to
avoid this problem of slaves sensing too old data without a good reason.
However this max data validity time is likely a setting that should be
configurable by the Redis Cluster user in a way completely independent
from the node timeout.

This commit makes it simple to start an handshake with a specific node
address, and uses this in order to detect a node IP change and start a
new handshake in order to fix the IP if possible.

As specified in the Redis Cluster specification, when a node can reach
the majority again after a period in which it was partitioend away with
the minorty of masters, wait some time before accepting queries, to
provide a reasonable amount of time for other nodes to upgrade its
configuration.

This lowers the probabilities of both a client and a master with not
updated configuration to rejoin the cluster at the same time, with a
stale master accepting writes.

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.

This prevented 32bit cluster instances from clearing the FAIL flag when
needed.

Ping sent and pong received fields need to be casted to long long to be
printed correctly into 32 bit systems.

80 cols friendly, some minor change to the code to make it simpler.

Now there is a function that handles the update of the local slot
configuration every time we have some new info about a node and its set
of served slots and configEpoch.

Moreoever the UPDATE packets are now processed when received (it was a
work in progress in the previous commit).

The commit also introduces detection of nodes publishing not updated
configuration. More work in progress to send an UPDATE packet to inform
of the config change.

After the change in clusterCron() frequency of call, we still want to
ping just one random node every second.

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.

When a slave requests our vote, the configEpoch he claims for its master
and the set of served slots must be greater or equal to the configEpoch
of the nodes serving these slots in the current configuraiton of the
master granting its vote.

In other terms, masters don't vote for slaves having a stale
configuration for the slots they want to serve.

The new API is able to remember operations to perform before returning
to the event loop, such as checking if there is the failover quorum for
a slave, save and fsync the configuraiton file, and so forth.

Because this operations are performed before returning on the event
loop we are sure that messages that are sent in the same event loop run
will be delivered *after* the configuration is already saved, that is a
requirement sometimes. For instance we want to publish a new epoch only
when it is already stored in nodes.conf in order to avoid returning back
in the logical clock when a node is restarted.

This new API provides a big performance advantage compared to saving and
possibly fsyncing the configuration file multiple times in the same
event loop run, especially in the case of big clusters with tens or
hundreds of nodes.

Previously the event was not logged but instead the node reported an
unknown packet type received.

We can accept a vote as long as its epoch is >= the epoch at which we
started the voting process. There is no need for it to be exactly the
same.