• mod_proxy_http2: simplifying implementation and reducing memory use by only
  running a single request at the time on a h2 backend connection. The previous
  implementation was just not good enough regarding flow control on the overall
  server and too complex compared to the performance benefits achieved. The goal
  with this change is to establish a solid base from which to optimize things
  again.
• Fixes slave connection input filter to use proper return code on speculative read
  encountering an EOF.
• Starting test_600 for mod_proxy_http2 use on server itself