[derekargueta]

graceful_shutdown sends both GOAWAYs that RFC 9113 §6.8 describes:

1. the first, with the last stream ID set to 2^31-1;
2. a PING;
3. the final GOAWAY, as soon as the PING is acknowledged.

A server therefore can't warn clients that a shutdown is coming and finish it later. That split is what proxies do when they drain. For example, Envoy sends the first GOAWAY as soon as a connection starts draining. Clients can then move new requests elsewhere, while requests already in flight are still accepted. Envoy sends the final GOAWAY only once its drain_timeout has passed.

This PR adds server::Connection::shutdown_notice(), which:

• sends the first GOAWAY (last stream ID 2^31-1, NO_ERROR) and the shutdown PING;
• does not send the final GOAWAY. That waits for a later graceful_shutdown(), which sends it:
  • at once if the PING has already been acknowledged, because the client has seen the notice;
  • otherwise, when the acknowledgement arrives, as graceful_shutdown does today;
• keeps accepting new streams;
• keeps an idle connection open until graceful_shutdown() is called;
• does nothing if a GOAWAY has already been sent.

When shutdown_notice isn't used, graceful_shutdown behaves exactly as before.

Implementation: GoAway tracks whether the notice is Sent or Acked. While a notice is outstanding, the shutdown PING's acknowledgement records that the client has seen it instead of sending the final GOAWAY.

Tests: two new tests in tests/h2-tests/tests/server.rs:

• shutdown_notice_then_graceful_shutdown: the notice and its PING are sent, and the connection keeps taking streams after the acknowledgement. Then graceful_shutdown sends GOAWAY(3) without a second PING.
• graceful_shutdown_before_shutdown_notice_acked: repeated shutdown_notice and graceful_shutdown calls send one GOAWAY and one PING. The final GOAWAY follows the acknowledgement, and a stream that was in flight still completes.

Related: hyperium/hyper#3961 (a different shutdown sequence, with the same goal of telling clients early).

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