lib: nb: add list_entry_done() callback to free resources

lib: nb: add list_entry_done() callback to free resources

The existing iteration callback only allows for a daemon to return a
pointer to objects that must already exist and must continue to exists
indefinitely.

To allow the daemon to return allocated iterator objects and for locking
it's container structures we need a callback to tell the daemon when FRR
is done using the returned value, so the daemon can free it (or unlock
etc)

That's what list_entry_done() is for.

Signed-off-by: Christian Hopps <chopps@labn.net>