StatisticsHandler broken for async applications

[sbordet]

Jetty version
10.0.x

Description
In Jetty 10, StatisticsHandler.handle() has a finally block where it flushes the response if _shutdown != null, which is always true (unless stopped).

This is bad because async applications get their response flushed (with all the headers) before they even get a chance to add headers.

[gregw]

The test should at least be: if (shutdown != null && shutdown.isShutdown()), but I don't understand why we need the flush buffer at all... nor how this current code is not failing some test or other?

[lachlan-roberts]

Fixed with PR #5517.