Bug 611 - BGP Update message serialization
- serializers use ByteBuf instead of byte[]
- added update message path attributes serialization
- AttributeSerializer expects ByteBuf as method param
- MessageSerializer uses ByteBuf as method param

Change-Id: Ic7bb94e1ab20980a90967ef0a8b00ea17d04db69
Signed-off-by: Martin Bobak <mbobak@cisco.com>
Signed-off-by: Dana Kutenicsova <dkutenic@cisco.com>