mod-h2 v1.15.11

• 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]