extend DnsResolve to support ipv6

[romange]

Some environments do not rely on DNS servers for dns resolution. It seems that we do not support this case.

Expected: /etc/hosts would be taken into account.

a few leads to investigate:

• https://c-ares.org/ares_init_options.html has char *hosts_path;
• there is also a pure ares_gethostbyname_file call.

Seems that the first option would be a cleaner solution.

[romange]

@dranikpg update: the problem seems to be related to binding ipv6 network interface and not to /etc/hosts. to reproduce:

1. ./dragonfly --logtostderr --bind ::1 --port 6379
2. ./dragonfly --logtostderr --bind ::1 --port 6380
3. redis-cli -h ::1 -p 6380

   [::1]:6380> replicaof ::1 6379
   (error) ERR could not resolve master dnsNo route to host

   the problem started with v1.16 because we switched to DnsResolve in replication

[dranikpg]

Closed with #247