Add an optional idle timeout for downstream HTTP/2 connections

[acoshift]

The downstream H2 accept loop (accept_downstream_sessions) only ends when the client closes the connection, a codec error occurs, or the runtime graceful-shutdown ceiling fires. A client that completes the handshake and then sends no streams keeps the whole connection alive indefinitely.

Add an opt-in HttpServerOptions::h2_idle_timeout. When set, a connection that has accepted no new stream and has no request in flight for that duration is closed. In-flight streams are tracked via a StreamGuard handed to the per-session callback. The idle timer is re-armed each loop iteration, so any stream activity resets it; on fire with nothing in flight the loop returns, dropping the connection and closing the socket immediately.