lib: Impelement the `*_del` list API.

lib: Impelement the `*_del` list API.

The new list api did not implement the `*_del` endpoint as
it was described in the docs here:

http://docs.frrouting.org/projects/dev-guide/en/latest/lists.html#c.Z_del

This patch implements the endpoints to return the object deleted if
found, otherwise NULL for all but the atomic lists.

The atomic list `*_del` code is marked as TODO and will remain undefined
for now.

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