lib, zebra: Fix last write command written

lib, zebra: Fix last write command written

With commit: a9ff90c41b0a95195d19d451ee83eb460e1599d0
the vrf_id_t was changed from a uint16_t to a uint32_t

Zebra tracked the last command sent to it's peer via peeking
into the data it was sending to each client ( since we had
lost the idea of what the command was when it was time to track
the data ).

Add a define to track this and add a bit of verbiage
to the code to allow us to notice when we screw with
the header again so that this is just fixed correctly
when it happens again.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>