Allow move-only handlers

[Marton <marton78@…>]

(See also ​http://stackoverflow.com/questions/17211263)

The requirement that handlers be copy constructible doesn't allow for the following idiom:

void connection::send_response()
{
    // block until previous response is sent
    std::unique_lock<std::mutex> locker(response_mutex_);

    // prepare response
    response_ = "foo";

    // send response back to caller. move the unique_lock into the binder
    // to keep the mutex locked until asio is done sending.
    asio::async_write(stream_,
                      asio::const_buffers_1(response_.data(), response_.size()),
                      std::bind(&connection::response_sent, shared_from_this(),
                                _1, _2, std::move(locker))
                      );
}

void connection::response_sent(const boost::system::error_code& err, std::size_t len)
{
    if (err) handle_error(err);
    // the mutex is unlocked when the binder is destroyed
}

Please remove this restriction, there is no reason for it when using C++11.

[cristi.cobzarenco@…]

Any progress/discussion on this? There's plenty of reasons to allow move-only constructors, another is to use stackless coroutines with moveable only members, such as a deadline_timer, without having to wrap everything in a shared_ptr, needlessly complicating the code for a coroutine.