Add configuration property for Reactor Netty shutdown quiet period

[bclozel]

As of reactor/reactor-netty#860, we can now configure the quiet period after the shutdown of the Reactor Netty web server.
We should consider using that API in NettyWebServer and even change the default value for a shorter one when devtools is present: right now, restarting the application with devtools inflicts the default quiet period on the user, making the feedback loop longer than necessary.

[Lacritz]

Hi @bclozel,
is this issue currently still unassigned? I would like to contribute here - but currently not seeing the newly introduced api reflected in the current dependencies. Am i missing something?

[bclozel]

@Lacritz we need to address spring-projects/spring-framework#24538 first.

[bclozel]

This has been superseded by #4657