This revises the new P2P `Transport` interface and does some preliminary code cleanups and simplifications.
The major change here is to add `Connection.Handshake()` for performing node handshakes (once the stream transport API is implemented, this can be done entirely independent of the transport). This moves most of the handshaking logic into the `Router`, such as prevention of head-of-line blocking, validation of peer's `NodeInfo`, controlling timeouts, and so on. This significantly simplifies transports, completely removes the need for internal goroutines, and shares common logic across all transports. This also allows varying the handshake `NodeInfo` across peers, e.g. to vary `ListenAddr`. Similarly, connection filtering is also moved into the switch/router so that it can be shared between transports.