zebra: Implement call back for route install/delete success/fail

zebra: Implement call back for route install/delete success/fail

When a route is installed or deleted into the kernel allow a
callback mechanism to handle the success/failure of
the kernel call.

This separation is to allow us to do these things:

1) In the future create a true pthread to handle route
install/deletes.  This way we can schedule these
events in a smarter fashion

2) Allow us to use a common southbound api for route
install and deletion.

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