feat: refactor peer class

[wemeetagain]

Resolves #54 and #56

• Peer has been renamed/replaced with PeerStreams class that manages incoming and outgoing streams.
• _addPeer and _removePeer have been slightly modified to suit - _removePeer closes a peer's streams
• getSubscribers now uses this.topics rather than peer.topics to determine the subscribers for a topic

PeerStreams manages:

• id - the PeerId of the peer
• protocol - the protocol negotiated with the peer (previously was an array of protocols, which is described in Outgoing peer connections - peer protocols #54 )
• _rawOutboundStream (previously named this.conn) - this is the stream returned from conn.newStream
• outboundStream (previously named this.stream) - this is the pushable stream to write outgoing messages to. Data pushed here is sent to _rawOutboundStream after first being transformed with a length-prefix encoding transform.
  It now manages an inbound stream as well
• _rawInboundStream - this is the stream returned in the callback attached to registrar.handle. Called when a peer dials us.
• inboundStream - this is an abortable stream with first a length-prefix decoding tranform applied. This is the stream that should be read from to receive incoming rpc messages.
• async attachOutboundStream(stream) - method to attach a new outgoing stream for management. If the stream is the first outgoing stream to be attached, this method triggers a stream:outbound event. If there is already an existing outgoing stream, the existing stream is gently shutdown (without triggering a close event) first, then the new stream is attached.
• attachInboundStream(stream) - method to attach a new incoming stream for management. If the stream is the first incoming stream to be attached, this method triggers a stream:inbound event. If there is already an existing incoming stream, the existing stream is gently shutdown (without triggering a close event) first, then the new stream is attached.
• write(data) - convenience method to push data to the outboundStream, if it exists. If it doesn't, this method throws an error.
• close() - method to shutdown both incoming and outgoing streams, triggers a close event. In conjunction with the base Pubsub impl, the close event will _removePeer.

[vasco-santos]

This is great!!

[vasco-santos]

Suggested change

	* @param {Stream} props.stream
	* @param {IterableStream} props.stream

Per https://gist.github.com/alanshaw/591dc7dd54e4f99338a347ef568d6ee9, we should call this an IterableStream. This is not a node stream and might create confusion

[wemeetagain]

I called it DuplexIterableStream so its known to be a duplex object ({ sink, source })