zebra: Add code to track sequence number from zebra_router

zebra: Add code to track sequence number from zebra_router

The sequence number used should be unique and increase by 1
for netlink commands.  This will allow the code to match
up batched commands to actual requests, so that we can signal
the failure correctly back.

So start the movement and tracking of sequence numbers as
an atomic uint32_t in zebra_router.  Modify the dataplane
code to start tracking contexts from this value.

In future commits we will move more of the sequencing
data into using this value.

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