zebra: revise redistribution delete to improve update case

zebra: revise redistribution delete to improve update case

When selecting a new best route, zebra sends a redist update
when the route is installed. There are cases where redist
clients may not see that redist add - clients who are not
subscribed to the new route type, e.g. In that case, attempt
to send a redist delete for the old/previous route type.

Revised the redist delete api to accomodate both cases;
also tightened up the const-ness of a few internal redist apis.

[7.2 version]

Signed-off-by: Mark Stapp <mjs@voltanet.io>