• Fixed #253 where requests were not logged and accounted in a timely
  fashion when the connection return to "keepalive" handling, e.g. when
  the request served was the last outstanding one.
  This led to late appearance in access logs with wrong duration times
  reported.
• New directive 'H2EarlyHint name value' to add headers to a response,
  picked up already when a "103 Early Hints" response is sent. 'name' and
  'value' must comply to the HTTP field restrictions.
  This directive can be repeated several times and header fields of the
  same names add. Sending a 'Link' header with 'preload' relation will
  also cause a HTTP/2 PUSH if enabled and supported by the client.
• accurately report the bytes sent for a request in the '%O' Log format.
  This addresses #203, a long outstanding issue where mod_h2 has reported
  numbers over-eagerly from internal buffering and not what has actually
  been placed on the connection.
  The numbers are now the same with and without H2CopyFiles enabled.