librados: don't destroy ObjListCtx when iterator reaches the end

librados: don't destroy ObjListCtx when iterator reaches the end

There's no need to destroy it when we've used it once. It'll get destroyed
by the destructor anyway. Now that there's a seek method, we might want seek
and iterate again anyway, which is not possible if the ObjListCtx is gone.

Change get_next() to check the underlying ObjListCtx for the end of iteration
instead of deleting ObjListCtx, and modify the equality operator to count
the end marker (with ctx == NULL) as equal to having a ObjListCtx at the
end of its list.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>