(Tarball) Enhance DnsResolver

[fraxken]

Improve DnsResolver class to implement:

• Cache to avoid trigerring multiple lookup for the same host
• Use dns.resolve4 and dns.resolve6 to avoid limited libuv thread pool bottleneck with lookup: see https://nodejs.org/api/dns.html#implementation-considerations

There is also room to enhance the implementation inside of NpmTarball class (quite ugly right now).