The Inbound service only needs the network setup for some requests, but
it can service other requests without it. Making it return
Poll::Pending until the network setup finishes means that initial
network connections may view the Inbound service as overloaded and
attempt to load-shed.
The original version of this commit ran into
https://github.com/rust-lang/rust/issues/64552
again. Thanks to @yaahc for suggesting a workaround (using futures combinators
to avoid writing an async block).
Remove the seed command entirely, and make the behavior it provided
(responding to `Request::Peers`) part of the ordinary functioning of the
start command.
The new `Inbound` service should be expanded to handle all request
types.