• Integrated changes from Apache trunk.
• Log requests and sent the configured error response in case of early detected
  errors like too many or too long headers. [Ruediger Pluem]
• added test for buffering across a mod_http2+mod_proxy_http2 connection
• added TRACE2 logging to show buffering flag when writing task output
• new option 'H2OutputBuffering on/off' which controls the buffering of stream output.
  The default is on, which is the behaviour of older mod-h2 versions. When off, all
  bytes are made available immediately to the main connection for sending them
  out to the client. This fixes interop issues with certain flavours of gRPC, see
  als #207.
• Eliminated some Python deprecation warnings in test code.