zebra: add interface address apis for dplane

zebra: add interface address apis for dplane

Add new apis for dplane interface address handling, based on
the existing api. The existing api is basically split in two:
the first part processes an incoming netlink message in the
dplane pthread, creating a dplane context with info about
the event. The second part runs in the main pthread and uses
the context data to update an interface or connected object.

Signed-off-by: Mark Stapp <mjs.ietf@gmail.com>