DNS SRV: allow workaround for IPv6 nameserver

The Bootstrap class, used to bootstrap the SDK from a DNS SRV record, relies on the default host machine DNS resolution.

In the case where the DNS server list configured in the host machine include IPv6 entries, the whole initialization breaks and there's no way of using a different resolution process.

It should at least be possible to set an explicit dns://XXXX value for the java.naming.provider.url parameter used for the InitialDirContext construction, where XXXX is a name server's IP (IPv4, of course) provided by the user as a workaround, for cases where the machine's configuration itself cannot be easily modified.