Rework NETCONF interfaces

NetconfSession and NetconfClientSessionListener are supposed to be
interfaces, not classes. Convert them to such and introduce utility
classes which retain their current functionality. Remove an unused
method and fix type safety warnings while we're at it.

Change-Id: Id7d78c831e3c3d46abb4379efe4a5ca353cd55ff
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Rework NETCONF interfaces"