zebra: Adjust nhg handling for dataplane result off on shutdown

zebra: Adjust nhg handling for dataplane result off on shutdown

Now with this patch we can't use shutdown for cleanup:

```
commit 2fc69f03d2f49fcf34948e82e865cd302ae08da0 (pr_5079)
Author: Mark Stapp <mjs@voltanet.io>
Date:   Fri Sep 27 12:15:34 2019 -0400

    zebra: during shutdown processing, drop dplane results

    Don't process dataplane results in zebra during shutdown (after
    sigint has been seen). The dplane continues to run in order to
    clean up, but zebra main just drops results.

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

Adjusted nhg uninstall handling to clear data and other
cleanup before sending to the dataplane.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>