lib, bgpd: Refactor vrf handling through zclient

lib, bgpd: Refactor vrf handling through zclient

Protocols receive zclient vrf creation events from zebra.
This data was being handed to the protocol to decode and
then to hand back to zclient to create the vrf to then
handle appropriately.  This is a bad idea.

Modify the code such that when zclient.c receives a vrf
event from zebra that it decodes the data and just creates
the vrf.  Individual protocols just need to handle the
appropriate vrf events.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>