Feature: show h2 threads in scoreboard

[covener]

It seems like we need a way to do one of:

• move the threadpool into something managed by the core or MPM and include extra scoreboard handles at creation time (this happens really early though)
• ask the MPM's to schedule and run the h2 threads (probably easiest in event)
• find a way for a module to ask for an over-allocation the scoreboard handles AND coordinate thread offsets.

This would allow the h2 threads to have their actual slots int he scoreboard. Currently they all share the original connection processing threads scoreboard handle and mockup some event internal structures

[icing]

I had a closer look at scoreboard and what it does. And looked at /server-status after some loadtesting. Seeing threads that go into 'G' and never leave that state. Does not look ok, afaik.

As first mitigation, added some ap_update_child_status() into my main I/O handling. But that does not solve the problem.

For the 2.4 release, I need a way for mod_h2 to co-exist with its own threads, since I think substantial changes in MPMs are not going to fly.

[icing]

So, the scoreboard (mod-status) can be extended, as see by balancers and ssl. Use this extension mechanism to display the status of all active http2 workers. Show request being processed and progress.