Re-write interface to buffer streams internally

[thomaseizinger]

Once #150 is solved, we more or less have back-pressure in yamux. See libp2p/specs#471 (comment) for an explanation.

To make sure applications are compliant with this, we should buffer the inbound streams internally. This allows us to align the buffer size with the ACK backlog.

The interface I'd like to see is:

impl Connection<S> {
    /// Allow the connection to make progress by sending pending frames and reading data from the socket.
    ///
    /// This will buffer up to 256 new inbound streams and immediately reset any additional ones.
	pub fn poll(&mut self, cx: &mut Context<'_>) -> Poll<Result<()>>;

    /// Attempt to open a new outbound stream.
    ///
    /// This will suspend in case we already have 256 unacknowledged outbound streams.
	pub fn poll_new_outbound(&mut self, cx: &mut Context<'_>) -> Poll<Result<Stream>>;

    /// Return the next inbound stream.
    ///
    /// This will return streams from an internal buffer that gets populated as part of [`Connection::poll`].
    /// In case, the buffer is empty, this will return [`Poll::Pending`].
	pub fn poll_next_inbound(&mut self, cx: &mut Context<'_>) -> Poll<Result<Stream>>;
}

[thomaseizinger]

I've tried this and I am not sure it is worth the refactoring, will put on hold.

The advantage of the current interface is that it is quite easy to determine when the connection is closed: poll_next_inbound returns None.

With the above interface, it is not obvious that this is what we should do. Because we have ConnectionError::Closed, it is equally feasible for all the above operations to now fail in case the connection is closed.