BUG-1116: Rework AbstractAdjRIBsIn interface

Rework the class such that we have support for encoding of individual
NLRI types, as well as adding proper insulation of the subclasses from
the actual place where a particular data node is being written to.

Change-Id: I82196d506004c30745820fd2769d63d34d5cf806
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Milos Fabian <milfabia@cisco.com>