sharpd: Start infrastructure to allow for redistribution testing

sharpd: Start infrastructure to allow for redistribution testing

Start the work in sharpd to allow the testing of redistribution
of routes.  Namely telling zebra to tell us about redistribution events
via the callback.

Future work here will allow sharpd to specify the redistribution
events it wants and to allow us to track that via counters.

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