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.