Datagram channel redesign

[bjoerntm]

Address handling in the datagram layer. Here's the channel handling I have in mind:

• unidirectional datagram channels (as there is no session, there is no reason to expect datagrams to be related)
• during creation of a sending channel, the receiver address and port are set
• to send to different addresses, higher protocol layers create new datagram channel instances

[bjoerntm]

This would also indicate a different handling of the "counterpartAddress" in both the DatagramMessage and the SessionMessage. The packets themselves are mostly oblivious of the address (with the exception that we may want to extract the sender from a received UDP packet in specific scenarios).

One aspect we should have in mind, though, is whether we want to design a debugging interface.