Fixes #8224.

Small change without tests which should be OK.
Writing a unit test for this is a bit tricky
because it may need a custom Accumulator to
test properly. However I think it's OK to make
this change so long as no existing unit tests
fail.

Fixes #8368.

Refactor the AkkaHttpServer class so that more of its internal
behavior is accessible to advanced users. Users can now extend
this class and override behavior through protected methods.
The custom class can be hooked into Play through the existing
play.server.provider setting.

By allowing users to change the Akka HTTP server behavior,
particularly the server settings, we reduce the pressure on
Play to provide a perfect and universal configuration out of
the box. We can now provide a configuration that is right for
most users and allow advanced users to reconfigure their
settings as needed.

For example, it is now possible to modify the Akka HTTP server's
configuration to extend the list of HTTP methods that is
supported. This example is shown in an integration test.

Upgrade to Netty 4.1.25.Final.