Merge of r1767936,1768160,1769192,1769550 from trunk:

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.
     adding support for newly proposed 103 status code.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1769595 13f79535-47bb-0310-9956-ffa450edef68