p2p: Implement Supervisor

[xla]

Work In Progress

This is a placeholder PR to hold all the outstanding changes for the p2p work, will be broken apart further.

Stacked on top of #904

[thanethomson]

I've been looking through the abstractions again in the p2p/src/transport.rs module, and wanted to ask if there's a particular reason why "read/write" and "send/receive" naming conventions are mixed in the P2P architecture?

For example, the Connection trait has StreamRead and StreamSend associated types (instead of StreamRead/StreamWrite or StreamSend/StreamReceive).

[xla]

For example, the Connection trait has StreamRead and StreamSend associated types (instead of StreamRead/StreamWrite or StreamSend/StreamReceive).

In order to distinguish physical connection semantics I'm keen on using send/receive for streams. The inconsistency you are pointing out is definitely an oversight and needs to be addressed.