Imporovements and fixes

* Thread safety
* Proper SCTP stream ID handling
* ...and improvements for better software quality

Safety improvements

Various improvements

Bugfix: Heartbeat handling.

Bug fixes in message length

Improve robustness and introduce fuzzing with go-fuzz

Completed SSNM messages and small fixes in documents'

Completed implementation of SSNM messages.

Renamed codec funcs to follow conventions.

Bug fixes