*) mod_proxy_http2: the "ping" proxy parameter

  *) mod_proxy_http2: the "ping" proxy parameter
     (see <https://httpd.apache.org/docs/2.4/mod/mod_proxy.html>) is now used
     when checking the liveliness of a new or reused h2 connection to the backend.
     With short durations, this makes load-balancing more responsive. The module
     will hold back requests until ping conditions are met, using features of the
     HTTP/2 protocol alone. [Ruediger Pluem, Stefan Eissing]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1878433 13f79535-47bb-0310-9956-ffa450edef68