Refactor Additional header for netconf hello message.

Allow additional header only for hello messages.
Introduce customized (en|decoders) for hello message to parse/serialize additional header.
These handlers are replaced after successful negotiation.

Change-Id: I54c441bc166e7141f503888f0eb1934882971045
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>
Merge "Refactor Additional header for netconf hello message."