Sentinel role support

[melo]

This is still work in progress. It works, but there is stuff missing.

In particular:

• tests: not sure how to test this stuff yet;
• corner case of wrong role detected: we connected during a demotion or promotion of the server we connected to, and the role no longer matches what we expected - we need a clean way to go through the reconnection process.

The first item should be easy, I just need some time with the sentinel tests to understand them well.

The second, I think the best bet is make the code path for Sentinel-based connections to be the same as the "reconnect on" path. We get reconnects for free if we have the wrong server Role, and the build sock stuff already takes care of going through Sentinel to pick up a new server.

Anyway, please review…

Test script during development:

#!/usr/bin/env perl

use 5.014;
use warnings;
use lib 'lib';
use Redis::Sentinel;
use Data::Dumper;

my $r = Redis->new(
  sentinels => ['127.0.0.1:2001', '127.0.0.1:2002', '127.0.0.1:2003'],
  service   => 't',
  role      => 'slave',
);

my $i = $r->info('replication');
warn "from $r->{server}: " . Dumper($i);

[melo]

Added some tests, I'll see the reconnect tomorrow...

[melo]

@dams care to review this? :)

[dams]

at first glance it looks good to me, I'll have a detailed look a bit later. Thanks for the work !