zebra: refactor netlink route message parsing

zebra: refactor netlink route message parsing

Separate core netlink route message parsing into a new api that
uses a dplane ctx to hold the parsed attribute data. Use the new
api in two paths: the normal netlink update message parsing path,
and in the FPM plugin, which also uses netlink encoding. The FPM
route-notificatin code runs in its own pthread, and only needs
a subset of the route info that zebra ordinarily develops. This
change stops that pthread from accessing zebra's internal
data, such as vrfs and ifps, that are not thread-safe.

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