Wrong assumptions on SCTP data chunk and design issue..

In one SCTP package we can have _multiple_ SCTPData chunks. The current code doesn't support it and will provide all other chunks as payload. I removed the special case of decodeSCTPChunk and then put a Payload into the SCTPData struct. One issue is that the test data is kind of invalid...

The fundamental issue is that the structure of SCTP is like this.

```
IP
SCTP
SCTPDataChunk
Payload
SCTPDataChunk
Payload
```

While p.NextDecoder will just be linear. We would need to have some kind of special "Payload" so we can jump to the next SCTPDataChunk. Alternatively be always eager for SCTP...