mod_http2: push diary to avoid duplicate pushes, cache-digest handling,...

mod_http2: push diary to avoid duplicate pushes, cache-digest handling, http2-status handler for stats about current http2 connection

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1724820 13f79535-47bb-0310-9956-ffa450edef68