Implement move-based async_accept()

[gegles]

Draft PR to converse about #2

[gegles]

@breese this is the best i could do so far.... It compiles and seems to accept the first socket but then I get runtime exception from socket.ipp:316

        receive_output_queue.emplace(std::move(operation));

I haven't dug much into your code yet, but I suspect that you keep track of the socket by their address pointer which is now disrupted by moving the socket around....

I also had to "fix" socket move-constructor logic for things to work (with the shared_ptr).

Appreciate any help to put me on the right track...

[breese]

Your intuition about addresses is correct.

The multiplexer keeps track of the mapping between remote endpoints and datagram::sockets, and it uses the a non-owning pointer for that. The socket is responsible for adding and removing itself.

The remote endpoint only becomes available when the acceptor detects a remote connection, at which point we set the multiplexer on the socket. This lazy construction means that the socket destructor must check if multiplexer has been set.

Now, what happens is that the socket move constructor moves the multiplexer leaving the moved-from socket with an empty multiplexer, so when the destructor of the moved-from socket is called it does not remove itself from the multiplexer because it has none.