Add IPv6 Server2Server support

This commit adds experimental server2server support.

It will make DJabberd::DNS prefer AAAA records over A records
and implement IPv6 capabilities for dialback and outgoing
component connectinos.

There are also some new testcases, especially for DJabberd::DNS.

The DJabberd::DNS tests use a mock-up DNS server for reliable
and reproduceable DNS tests.

Please note that it is still very much experimental.

Added Test::SharedFork to make the DNS test pass.