Integrate h2 workers into core MPM infrastructure

[icing]

Current Status

The threads processing individual http2 streams are kept in a separate worker pool. This excludes them from the common Apache MPM infrastructure. Configurational parameters, such as MaxThreadsPerChild do not apply to them. They do not show up on the scoreboard (see also issue #23).

Proposal

Make htttp2 workers part of the Apache MPM infrastructure by using threads managed by MPM. This requires modifications of the MPM itself, and very likely impact the specific MPM modules such as event, worker, prefork and others.