Allow accepting new connections during a graceful shutdown #3961

Is your feature request related to a problem? Please describe.
We want a very specific behavior for graceful shutdowns in our hyper-based proxy. We set two deadlines, min (typically 5-10s), and max (could be much longer). We want the following sequence during shutdown:

• t=0s: send GOAWAY on all HTTP2 connections and start responding with connection: close on all http1 requests. Continue to accept new connections, and handle them the same (GOAWAY/connection: close) them.
• After min deadline: stop accepting new connections. Continue existing connections, with the same GOAWAY/connection: close.
• After max deadline: shutdown everything.

The reason for the time when we accept new connections, but tell clients to go away, is that there is some time for the load balancers in front of the proxy (this could be something like AWS NLB, Kubernetes kube-proxy, etc) to de-register the proxy instance and stop sending traffic to it. So immediately dropping new connections results in downtime. However, during this time its nice to start to encourage clients to goaway, since its likely they will select another proxy instance (either because the fronting LB already de-registered the old instance (the min deadline is intended to be the worst case, so usually its much faster), or because of random chance it selects a different one).

Today, the best we can do is wait until after min deadline and stop accepting new connections and start GOAWAY/connection: close at the same time.

Describe the solution you'd like
A new flag on hyper_util::server::conn::auto, and on hyper::server::conn::http1 to enable the "different graceful shutdown mode" (naming TBD!). This would change:

• hyper/src/proto/h1/dispatch.rs

  Line 97 in f9f8f44

  if self.conn.is_write_closed() || self.conn.has_initial_read_write_state() {

  - skip the close() call

• hyper/src/proto/h1/conn.rs

  Line 878 in f9f8f44

  self.state.close();

  - skip the close() call
• https://github.com/hyperium/hyper-util/blob/66afc93debef02548c86e8454e6bc01cf4fca280/src/server/conn/auto/mod.rs#L341 - do no error here
• https://github.com/hyperium/hyper-util/blob/66afc93debef02548c86e8454e6bc01cf4fca280/src/server/conn/auto/mod.rs#L525-L531 -- if cancelled, then call graceful_shutdown() on the newly served connections (and do the same for UpgradeableConnection)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

If these changes are acceptable I would be happy to contribute both parts of the change