Relayed peer: cutoff sending messages to itself

If a relayed peer wants to make a request to itself (e.g. when putting or getting data), it always contacted the relay peer which again sent it back to the requester (same peer)...

I fixed it by introducing a new <code>SendType#SELF</code>. It's detected when the sender and receiver peer have the same peerID.

This pull request also contains other minor fixes and documentation improvements.