Review WebServers' auto-start behavior

[wilkinsona]

Auto-start was introduced before 1.0. The intent appears to have been to prevent the server from starting. At the time it was when the port was set to 0. It's now when the port is set to a negative value. ServletContextAware is also mentioned which suggests that such components should still be called with the current ServletContext.

I think the name autoStart is misleading. It doesn't control whether the web server will be started automatically, but what start will do. Put another way, calling start() on a web server with auto-start disabled does not put it in the same state as one that was created with auto-start enabled.

We have four web servers now, and various behaviors when auto-start is disabled:

WebServer	Auto-start disabled behavior
NettyWebServer	No auto-start support
JettyWebServer	Server isn't started at all
TomcatWebServer	Tomcat is started but the connector's protocol handler is stopped
UndertowWebServer	Undertow instance isn't even created

With the opportunity that 4.0 provides for breaking changes, we should review the auto-start behavior. Given the inconsistency of the current implementations we should consider if the feature's needed at all. It also doesn't appear to be documented so it perhaps isn't widely used. If we want to keep it, we should, if possible, offer consistent behavior across all web servers. We should also consider renaming it.

[wilkinsona]

We should look at #17073 at the same time as there's quite a bit of overlap.