Redis-tools is a connection of tools no longer mantained that was
intented as a way to economically make sense of Redis in the pre-vmware
sponsorship era. However there was a nice redis-stat utility, this
commit imports one of the functionalities of this tool here in redis-cli
as it seems to be pretty useful.

Usage: redis-cli --stat

The output is similar to vmstat in the format, but with Redis specific
stuff of course.

From the point of view of the monitored instance, only INFO is used in
order to grab data.

This is needed in order to colorize it as next step.
We use conventions in output messages such as

>>> This is an action
*** This is a warning
[ERR] This is an error
[OK] That's fine

And so forth, so that a color will be associated checking the first
three chars.

Slot zero was hardcoded (!)

This way we are sure to destroy the slot->key map every time we destroy
the DB, for instance when reloading a DB due to replication.

Open slots are slots found in importing or migrating slot when a
cluster check is performed.

This way we make sure every time a master is turned into a replica
the flag will be cleared.

When a master turns into a slave after a failover event, make sure to
clear the assigned slots before setting up the replication, as a slave
should never claim slots in an explicit way, but just take over the
master slots when replacing its master.

A slave node set this flag for itself when, after receiving authorization
from the majority of nodes, it turns itself into a master.

At the same time now this flag is tested by nodes receiving a PING
message before reconfiguring after a failover event. This makes the
system more robust: even if currently there is no way to manually turn
a slave into a master it is possible that we'll have such a feature in
the future, or that simply because of misconfiguration a node joins the
cluster as master while others believe it's a slave. This alone is now
no longer enough to trigger reconfiguration as other nodes will check
for the PROMOTED flag.

The PROMOTED flag is cleared every time the node is turned back into a
replica of some other node.

Sender flags were not propagated for the sender, but only for nodes in
the gossip section. This is odd and in the next commits we'll need to
get updated flags for the sender node, so this commit adds a new field
in the cluster messages header.

The message header is the same size as we reused some free space that
was marked as 'unused' because of alignment concerns.

So when the failing master node is back in touch with the cluster,
instead of remaining unused it is converted into a replica of the
new master, ready to perform the fail over if the new master node
will fail at some point.

Note that as a side effect clients with stale configuration are now
not an issue as well, as the node converted into a slave will not
accept queries but will redirect clients accordingly.

The code handling a master that turns into a slave or the contrary was
improved in order to avoid repeating the same operations. Also
the readability and conceptual simplicity was improved.

It's just a simpler way to CLUSTER DELSLOTS with all the slots as
arguments, in order to obtain a node without assigned slots for
reconfiguration.

Redis Cluster can cope with a minority of nodes not informed about the
failure of a master in time for some reason (netsplit or node not
functioning properly, blocked, ...) however to wait a few seconds before
to start the failover will make most "normal" failovers simpler as the
FAIL message will propagate before the slave election happens.

Also, a few nearby comments improved.

See the function top-comment for info why this is useful sometimes.

Also don't check for NOADDR as we check that node->link is not NULL
that's enough.

That's trivial as we just need to increment the count of masters that
received with an ACK.

However the failover is yet not really performed.