bgpd: Type-2/MAC-IP SYNC route handling

SYNC routes are paths rxed from a local-ES peer. These routes result in
the installation of local dataplane entries i.e. with access port as
destination (vs. the remote-VTEP destination that results in the packet
being sent via the VxLAN overlay).

If a SYNC path is selected as the best path it is always turned around
into a local path which immediately lowers the status of the SYNC path
to non-best. However we need to keep track of the highest MM seq-number
and peer activity to continue advertising the local path. In order to
do that we need information from the "second-best" SYNC path to be
bubbled up to the local best path. This "SYNC" info is then consolidated
and sent to zebra which is responsible for the MM handling and local
path management.

Signed-off-by: Anuradha Karuppiah <[email protected]>