Redis 2.7.106 (2.8 Release Candidate 6).
Sentinel: always send CONFIG REWRITE when changing instance role.

This change makes Sentinel less fragile about a number of failure modes.

This commit also fixes a different bug as a side effect, SLAVEOF command
was sent multiple times without incrementing the pending commands count.
Sentinel: increment pending_commands counter in two more places.

AUTH and SCRIPT KILL were sent without incrementing the pending commands
counter. Clearly this needs some kind of wrapper doing it for the caller
in order to be less bug prone.
Sentinel: sentinelSendSlaveOf() was missing a var and the prototype.
Fix broken rdbWriteRaw() return value check in rdb.c.

Thanks to @PhoneLi for reporting.
Deprecate utils/redis-copy.rb in favor of redis-copy gem
Fixed typo in release candidate notes.
Log to what master a slave is going to connect to.
Sentinel: epoch introduced.

Sentinel state now includes the idea of current epoch and config epoch.
In the Hello message, that is now published both on masters and slaves,
a Sentinel no longer just advertises itself but also broadcasts its
current view of the configuration: the master name / ip / port and its
current epoch.

Sentinels receiving such information switch to the new master if the
configuration epoch received is newer and the ip / port of the master
are indeed different compared to the previos ones.
Sentinel: remove code not useful in the new design.
Sentinel: handle Hello messages received via slaves correctly.

Even when messages are received via the slave, we should perform
operations (like adding a new Sentinel) in the context of the master.
Sentinel: leadership handling changes WIP.

Changes to leadership handling.

Now the leader gets selected by every Sentinel, for a specified epoch,
when the SENTINEL is-master-down-by-addr is sent.

This command now includes the runid and the currentEpoch of the instance
seeking for a vote. The Sentinel only votes a single time in a given
epoch.

Still a work in progress, does not even compile at this stage.
Sentinel: epoch introduced in leader vote.
Sentinel: fix PUBLISH to masters and slaves.
Sentinel: allow to vote for myself.
Sentinel: wait some time between failover attempts.
Sentinel: +new-epoch events.
Sentinel: when starting failover seek for votes ASAP.
Sentinel: new failover algo, desync slaves and update config epoch.
Sentinel: added config-epoch to SENTINEL masters output.