Enforce back-pressure on incoming substreams using new `StreamMuxer` interface

[thomaseizinger]

Once https://github.com/libp2p/rust-libp2p/issues/2722 is complete, I would suggest we inline the HandlerWrapper component into the swarm::Connection struct.

This will resolve some indirection which:

• Allows us to use the incoming substream limit to enforce back-pressure
• Simplifies understanding of the code because StreamMuxer and ConnectionHandler are polled by the same component (Connection) and directly pass data between each other instead of going through another layer.