First implementation of Redis Sentinel.

This commit implements the first, beta quality implementation of Redis
Sentinel, a distributed monitoring system for Redis with notification
and automatic failover capabilities.

More info at http://redis.io/topics/sentinel
Typo.
Fixed an error in the example sentinel.conf.
Sentinel: more robust failover detection as observer.

Sentinel observers detect failover checking if a slave attached to the
monitored master turns into its replication state from slave to master.
However while this change may in theory only happen after a SLAVEOF NO
ONE command, in practie it is very easy to reboot a slave instance with
a wrong configuration that turns it into a master, especially if it was
a past master before a successfull failover.

This commit changes the detection policy so that if an instance goes
from slave to master, but at the same time the runid has changed, we
sense a reboot, and in that case we don't detect a failover at all.

This commit also introduces the "reboot" sentinel event, that is logged
at "warning" level (so this will trigger an admin notification).

The commit also fixes a problem in the disconnect handler that assumed
that the instance object always existed, that is not the case. Now we
no longer assume that redisAsyncFree() will call the disconnection
handler before returning.
Sentinel: check that instance still exists in reply callbacks.

We can't be sure the instance object still exists when the reply
callback is called.
Sentinel: changes to connection handling and redirection.

We disconnect the Redis instances hiredis link in a more robust way now.
Also we change the way we perform the redirection for the +switch-master
event, that is not just an instance reset with an address change.

Using the same system we now implement the +redirect-to-master event
that is triggered by an instance that is configured to be master but
found to be a slave at the first INFO reply. In that case we monitor the
master instead, logging the incident as an event.
Sentinel: debugging message removed.
Prevent a spurious +sdown event on switch.

When we reset the master we should start with clean timestamps for ping
replies otherwise we'll detect a spurious +sdown event, because on
+master-switch event the previous master instance was probably in +sdown
condition. Since we updated the address we should count time from
scratch again.

Also this commit makes sure to explicitly reset the count of pending
commands, now we can do this because of the new way the hiredis link
is closed.
Sentinel: reset pending_commands in a more generic way.
Sentinel: abort failover if no good slave is available.

The previous behavior of the state machine was to wait some time and
retry the slave selection, but this is not robust enough against drastic
changes in the conditions of the monitored instances.

What we do now when the slave selection fails is to abort the failover
and return back monitoring the master. If the ODOWN condition is still
present a new failover will be triggered and so forth.

This commit also refactors the code we use to abort a failover.
Some cleanup in sentinel.conf
Fix warning in redis.c for sentinel config load
Sentinel: ability to execute notification scripts.
comment fix

improve English a bit. :-)
Sentinel: don't start a failover as leader if there is no good slave.
Include sys/wait.h to avoid compiler warning

gcc warned about an implicit declaration of function 'wait3'.
Including this header fixes this.
Sentinel: scripts execution engine improved.

We no longer use a vanilla fork+execve but take a queue of jobs of
scripts to execute, with retry on error, timeouts, and so forth.

Currently this is used only for notifications but soon the ability to
also call clients reconfiguration scripts will be added.
Sentinel: abort failover when in wait-start if master is back.

When we are a Leader Sentinel in wait-start state, starting with this
commit the failover is aborted if the master returns online.

This improves the way we handle a notable case of net split, that is the
split between Sentinels and Redis servers, that will be a very common
case of split becase Sentinels will often be installed in the client's
network and servers can be in a differnt arm of the network.

When Sentinels and Redis servers are isolated the master is in ODOWN
condition since the Sentinels can agree about this state, however the
failover does not start since there are no good slaves to promote (in
this specific case all the slaves are unreachable).

However when the split is resolved, Sentinels may sense the slave back
a moment before they sense the master is back, so the failover may start
without a good reason (since the master is actually working too).

Now this condition is reversible, so the failover will be aborted
immediately after if the master is detected to be working again, that
is, not in SDOWN nor in ODOWN condition.
Sentinel: sentinel.conf self-documenation improved.
Sentinel: sentinelRefreshInstanceInfo() comments improved a bit.