Supporting async when filtering.

[magni-mar]

    recv(
        filter fil,
        std::set<control_packet_type> types,
        CompletionToken&& token
    ) {

I am trying to filter received packages by their type and was wondering why the function with package filtering doesn't support async as the normal recv function without filtering does support async.

    recv(
        CompletionToken&& token
    ) {

[redboltz]

duplicated #42