IPv6: Replace yang notifications

IPv6: Replace yang notifications

+ Replaced yang notifications with direct API call for reliability.
+ Removed duplicate code by replacing with Ipv6NaDecoder.
+ In ipv6-nd-util.yang, instead of "notification na-received",
  neighbor-advertise-packet is augmented with packet-metadata. The
  corresponding code has been updated.
+ This patch is dependent on Genius patch [0], hence it needs to be
  merged along with [0].

JIRA: NETVIRT-1434

[0] https://git.opendaylight.org/gerrit/#/c/76221/

Change-Id: Ib29c36a4a80f920729f69facd67bd5e78b3b6329
Signed-off-by: Somashekar Byrappa <somashekar.b@altencalsoftlabs.com>