Feature/jo/state managed udp

[Joannis]

Adds support for UDP clients and servers.

Motivation:

This change was made because UDP support was lacking on iOS. It's needed by my DNS client implementation, which I am in turn using for an iOS app I'm working on relying on SRV typed records.

Modifications:

• Adds a NIOTSDatagramListenerBootstrap for making UDP services
  • Adds a NIOTSDatagramListenerChannel that accepts UDP connections
• Adds a NIOTSDatagramChannel for UDP client connections
• Adds a NIOTSDatagramBootstrap that can create a new UDP client

Notes

This PR relies on #174 and #175, but it separated so that it's easy to review. This has been tested against my own libraries, but only features one test.

While I believe it to be working, I'm looking for feedback on how I can better test this implementation.

[Joannis]

@Lukasa updated

[Joannis]

Note that there were 2 small issues in the state managed channels that I've had to fix as well