feat: integrate multiplexing

[mattsse]

refactor some multiplex internals and add test for test protocol.

[Rjected]

I like the tests and updated inner type, just left some notes on extracting some parts of the poll loop to methods

[emhane]

is starting a SatelliteStream that much overhead when no extra protocols are shared that this enum is necessary?

[mattsse]

possibly, because this would involve routing through a channel,

[Rjected]

ok, this looks good, super excited to see multiple caps in practice

[mattsse]

After thinking a bit longer about this,
I'm going with this satellite approach over the MuxDemuxer because

• this still allows us to run a regular wrapped EthStream<TcpStream> most of the time (if no custom handlers installed and no matching additional caps) and we don't have any overhead routing through channels
• the SatelitteStream is convenient because it hides polling the other cap streams internally so the interface can be unified over the enum EthRlpxConnection

[emhane]

After thinking a bit longer about this, I'm going with this satellite approach over the MuxDemuxer because

• this still allows us to run a regular wrapped EthStream<TcpStream> most of the time (if no custom handlers installed and no matching additional caps) and we don't have any overhead routing through channels

MuxDemuxer doesn't route the main stream through channels

• the SatelitteStream is convenient because it hides polling the other cap streams internally so the interface can be unified over the enum EthRlpxConnection

polling the main stream that owns the MuxDemuxer polls the other cap streams