Use stream behaviour instead of request-response

[kalabukdima]

https://github.com/libp2p/rust-libp2p/tree/master/protocols/stream

• it doesn't require mutable Swarm for sending response
• it doesn't have an unnecessary limit for the number of connections
• simpler interface similar to a raw socket

[kalabukdima]

The stream protocol looks much better than request-response. It has a simpler implementation and a better interface which suits our needs just right. For example, it's easy to propagate the back pressure while still using it from multiple threads.
On the client side, you just request a connection to the peer and await the future to get the raw Stream which can be handled independently of every other stream, without locking the Swarm. So there is no need for any request caching inside the behaviour and matching request/response ids, making it much less error-prone.

Luckily for us, it's fully compatible on the wire with the request-response, so the transition will also be easy.

[kalabukdima]

Implemented in #145