Add distinct aggregate over node and relationships

[andyfengHKU]

This PR add support to distinct aggregate over nodes and relationships.

On the side, it refactors some of the legacy naming or inappropriate usage of smart pointers.

Note that I start using an additional wrapper on top of ku_dynamic_cast in this PR. See

    template<class TARGET>
    const TARGET* safePtrCast() const {
        return common::ku_dynamic_cast<const Expression*, const TARGET*>(this);
    }
    template<class TARGET>
    TARGET* unsafePtrCast() {
        return common::ku_dynamic_cast<Expression*, TARGET*>(this);
    }

@benjaminwinger and @ray6080 should comment regarding this wrapper.