• mod_http2: new directive 'H2PushResource' to enable early pushes before
  processing of the main request starts. Resources are announced to the
  client in Link headers on a 103 early hint response.
  All responses with status code <400 are inspected for Link header and
  trigger pushes accordingly. 304 still does prevent pushes.
  'H2PushResource' can mark resources as 'critical' which gives them higher
  priority than the main resource. This leads to preferred scheduling for
  processing and, when content is available, will send it first. 'critical'
  is also recognized on Link headers.
• mod_proxy_http2: uris in Link headers are now mapped back to a suitable
  local url when available. Relative uris with an absolute path are mapped
  as well. This makes reverse proxy mapping available for resources
  announced in this header.
  With 103 interim responses being forwarded to the main client connection,
  this effectively allows early pushing of resources by a reverse proxied
  backend server.
• mod_proxy_http2: adding support for 103 status code, as proposed by
  Kazuho Oku.