Use intrusive singly-linked attachables

By replacing the vector in `local_actor` with singly-linked attachables, we
save 16 Bytes on 64 bit machines. Relates #180.
`vector` => `forward_list` for pending responses

The `m_pending_responses` member of `local_actor` does not need random access.
Using a `forward_list` saves us another 16 Bytes. Relates #180.