bgpd: EVPN route handling

bgpd: EVPN route handling

Core EVPN route handling functionality. This includes support for the
following:
- interface with zebra to learn about local VNIs and MACIPs as well as
to install remote VTEPs (per VNI) and remote MACIPs
- create/update/delete EVPN type-2 and type-3 routes
- attribute creation, route selection and install
- route handling per VNI and for the global routing table
- parsing of received EVPN routes and handling by route type
- encoding attributes for EVPN routes and EVPN prefix creation (for
Updates)

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>