server: report why connections failed; Windows fd-exhaustion pause; review follow-ups

[iainmcgin]

Follow-ups deferred from the connection-layering stack (#306, #307, #308, #304).

let closed = server.serve_connection(stream, info, drain).await;
match (closed.reason(), closed.error()) {
    (CloseReason::Error, Some(err)) => metrics.connection_failed(err),
    (reason, Some(err)) => metrics.drain_failed(reason, err), // failed while winding down
    (reason, None) => metrics.connection_closed(reason),
}

• ConnectionClosed::error() / error_arc() return the error that ended a connection: always for CloseReason::Error, and for a failure while draining, whose reason stays the one that started the drain. A connection told to wind down before it picked a protocol reports no error: hyper-util returns an Interrupted "Cancelled" error there, which is matched by kind and message, so the workspace hyper-util floor is now 0.1.20 (checked against 0.1.19 and 0.1.20). ConnectionClosed keeps UnwindSafe / RefUnwindSafe through manual impls.
• The accept loop's pause after running out of file descriptors also matches Winsock's WSAEMFILE; before, the pause never applied on Windows.
• connectrpc::http re-exports the http crate, since the connection-extensions function takes http::Extensions.
• The h2 dev-dependency floor is 0.4.15, which the header-list-size GOAWAY test needs.