Merge of 1849296,1852038,1852101,1852339,1853171,1853967,1854365,1854963,1854964...

Merge of 1849296,1852038,1852101,1852339,1853171,1853967,1854365,1854963,1854964,1855295,1855411 from trunk:

  *) mod_http2: when SSL renegotiation is inhibited and a 403 ErrorDocument is
     in play, the proper HTTP/2 stream reset did not trigger with H2_ERR_HTTP_1_1_REQUIRED.
     Fixed. [Michael Kaufmann]

  *) mod_http2: new configuration directive: `H2Padding numbits` to control
     padding of HTTP/2 payload frames. 'numbits' is a number from 0-8,
     controlling the range of padding bytes added to a frame. The actual number
     added is chosen randomly per frame. This applies to HEADERS, DATA and PUSH_PROMISE
     frames equally. The default continues to be 0, e.g. no padding. [Stefan Eissing]

  *) mod_http2: ripping out all the h2_req_engine internal features now that mod_proxy_http2
     has no more need for it. Optional functions are still declared but no longer implemented.
     While previous mod_proxy_http2 will work with this, it is recommeneded to run the matching
     versions of both modules. [Stefan Eissing]

  *) mod_proxy_http2: changed mod_proxy_http2 implementation and fixed several bugs which
     resolve PR63170. The proxy module does now a single h2 request on the (reused)
     connection and returns. [Stefan Eissing]

  *) mod_http2/mod_proxy_http2: proxy_http2 checks correct master connection aborted status
     to trigger immediate shutdown of backend connections. This is now always signalled
     by mod_http2 when the the session is being released.
     proxy_http2 now only sends a PING frame to the backend when there is not already one
     in flight. [Stefan Eissing]

  *) mod_proxy_http2: fixed an issue where a proxy_http2 handler entered an infinite
     loop when encountering certain errors on the backend connection.
     See <https://bz.apache.org/bugzilla/show_bug.cgi?id=63170>. [Stefan Eissing]

  *) mod_http2: Configuration directives H2Push and H2Upgrade can now be specified per
     Location/Directory, e.g. disabling PUSH for a specific set of resources. [Stefan Eissing]

  *) mod_http2: HEAD requests to some module such as mod_cgid caused the stream to
     terminate improperly and cause a HTTP/2 PROTOCOL_ERROR.
     Fixes <https://github.com/icing/mod_h2/issues/167>. [Michael Kaufmann]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1855431 13f79535-47bb-0310-9956-ffa450edef68