net/http: the Server.Shutdown listener polling interval causes shutdown delay

[danp]

The fix for #33313 ensured that Shutdown waits for listeners to be closed but uses a fixed polling interval of 500ms. This causes Shutdown to almost always take at least 500ms which can be noticeable in interactive contexts.

It would be nice if the polling started with a small interval and backed off to 500ms or so.

[gopherbot]

Change https://golang.org/cl/264479 mentions this issue: net/http: use polling interval backoff in Server.Shutdown

[cagedmantis]

/cc @mikioh