Update EmbeddedWebApplicationContext so that the servlet container is
shutdown after the context is closed. Unfortunately shutting the
container down before the context has been closed causes exceptions if
the `/shutdown` actuator endpoint is used. It can also cause the Tomcat
classloader to throw IllegalStateExceptions if resources are accessed
during shutdown.

As this commit effectively reverts 0069e41c we need to fix the shutdown
deadlock issue reported in gh-4130 in a different way. The deadlock can
be caused when an incoming HTTP connection occurs whilst the context is
closing. The incoming connection triggers the `FrameworkServlet` to
call `initWebApplicationContext` which in turn calls `refresh`. The
`FrameworkServlet` checks `ApplicationContext.isActive()` before
performing an initialization but prior to this commit we would set
active to `false` before stopping the servlet container. We now override
`onClose` rather than `doClose` in `EmbeddedWebApplicationContext` to
ensure that the active flag is only set to `false` once the servlet
container has been stopped.

See gh-4130
Fixes gh-4396