Add Resolver interface and implementations

[Bouncheck]

Requested modification to allow for more control over hostname resolution when testing. Adds a set of Resolver classes and factories and hooks them into necessary places. See commit messages and javadoc for more details.

[Bouncheck]

I think this might be a little overzealous in replacing all found calls to InetAddress in core module, I'll probably revert codecs, clients etc. to direct calls. For our use case it's probably most important to plug it in ContactPoints class and metadata subdirectory

[Bouncheck]

Applied all suggestions with minor fixes.

[Bouncheck]

Changed logic of checking for "unresolvedness". Turns out Java 8 has different toString implementation than the version I used.