Resolve Bug:713 - Open snapshot only once per session.

Rework NetconfOperationServiceSnapshot lifecycle: Open it when
session negotiator is requested, thus at entry point where both
hello message and session listener is created. Close the snapshot
when NetconfSessionListener receives down/terminated event.

Change-Id: Id491110524ab635e552a49f48e4938dd7113d8bf
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>
Merge "Resolve Bug:713 - Open snapshot only once per session."