Parachain multiplexing

[maciejhirsz]

Companion PR to paritytech/substrate#7463

Expects id field on messages (defaults to 0 if absent), messages sent with different ids are treated as separate connections and can push the node to different chains.

• Check if we need to limit number of chains per node connector.
• Check if we need a way to broadcast a single message to all chains per node connector.

Unrelated: fixed continuation frames that sometimes errors on current build.

[dvdplm]

Looks like this will be fairly painless. :)

[dvdplm]

Unrelated to this PR, but is it ok that the backlog is unbounded? Maybe we could re-work this into a bounded queue?
EDIT: I see it is bounded to 10 below. Let's document that.

[maciejhirsz]

Yeah, it's bounded. I'm more worried about the number of ConnId being unbounded atm.

[dvdplm]

So this is the fix here?

[maciejhirsz]

The issue was that I had a separate backlog and Addr<Chain> per ConnId, but was using a single NodeId. Now the NodeId is tied to ConnId, everything else (using the enum for ConnMultiplex) is a refactor to make the intent clearer, and to avoid having 3 separate BTreeMaps for no good reason (one for chain address, one for backlog, one for node id).