Make TUniWeak out-streamable without weak_ptr or variant support.

Create two members, weak and unique_ptr, as I/O cannot handle variant. So we stream both.
O, and I/O cannot handle weak_ptr, but we really need it, so at least support writing (through THttpServer) by adding a `T*` with the same
value as the weak ptr, and have that streamed. This completely breaks the internal symmetry of TUniWeak, but it should get us started...