mod_http2/mod_proxy_http2: proper 100-continue handling up to backend, more...

mod_http2/mod_proxy_http2: proper 100-continue handling up to backend, more robustness on connection reuse with PING frames


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1765318 13f79535-47bb-0310-9956-ffa450edef68