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.

The new algorithm does not check replies time as checking for the
currentEpoch in the reply ensures that the reply is about the current
election process.

The old algorithm used a PROMOTED flag and explicitly checks about
slave->master convertions. Wit the new cluster meta-data propagation
algorithm we just look at the configEpoch to check if we need to
reconfigure slots, then:

1) If a node is a master but it reaches zero served slots becuase of
reconfiguration.
2) If a node is a slave but the master reaches zero served slots because
of a reconfiguration.

We switch as a replica of the new slots owner.

We need to:

1) Increment the configEpoch.
2) Save it to disk and fsync the file.
3) Broadcast the PONG with the new configuration.

If other nodes will receive the updated configuration we need to be sure
to restart with this new config in the event of a crash.

First change: now there is no need to be a master in order to detect a
failure, however the majority of masters signaling PFAIL or FAIL is needed.

This change is important because it allows slaves rejoining the cluster
after a partition to sense the FAIL condition so that eventually all the
nodes agree on failures.

The time is sent in requests, and copied back in reply packets.
This way the receiver can compare the time field in a reply with its
local clock and check the age of the request associated with this reply.

This is an easy way to discard delayed replies. Note that only a clock
is used here, that is the one of the node sending the packet. The
receiver only copies the field back into the reply, so no
synchronization is needed between clocks of different hosts.

Handshake nodes should turn into normal nodes or be freed in a
reasonable amount of time, otherwise they'll keep accumulating if the
address they are associated with is not reachable for some reason.