• Analyzing PR63170 more, mod_proxy_http2 needs to differentiate between its hosting
  stream gone and its master connection gone. The later is terminal, the former is not.
• mod_proxy_http2: ping the other side of an idle connection only when not already
  waiting on a PING response.
• Fixed an issue where a proxy_http2 handler entered an infinit loop when encountering
  certain errors on the backend connection.
  See https://bz.apache.org/bugzilla/show_bug.cgi?id=63170.