updated changes after mod_http2 backport from trunk

     be inherited by virtual hosts that define a CustomLog.

     [Edward Lu]

  *) mod_http2: connection how keep a "push diary" where hashes of already

     pushed resources are kept. See directive H2PushDiarySize for managing this.

     Push diaries can be initialized by clients via the "Cache-Digest" request

     header. This carries a base64url encoded. compressed Golomb set as described

     in https://datatracker.ietf.org/doc/draft-kazuho-h2-cache-digest/

     Introduced a status handler for HTTP/2 connections, giving various counters

     and statistics about the current connection, plus its cache digest value

     in a JSON record. Not a replacement for more HTTP/2 in the server status. 

     Configured as

     <Location "/http2-status">

         SetHandler http2-status

     </Location>

     [Stefan Eissing]

  *) mod_http2: Fixed flushing of last GOAWAY frame. Previously, that frame

     did not always reach the client, causing some to fail the next request.

     Fixed calculation of last stream id accepted as described in rfc7540. 

     Reading in KEEPALIVE state now correctly shown in scoreboard. 

     Fixed possible race in connection shutdown after review by Ylavic. 

     Fixed segfault on connection shutdown, callback ran into a semi dismantled session. 

     [Stefan Eissing]

  *) mod_http2: Added support for experimental accept-push-policy draft

     (https://tools.ietf.org/html/draft-ruellan-http-accept-push-policy-00). Clients

     may now influence server pushes by sending accept-push-policy headers.

     [Stefan Eissing]

  *) mod_http2: new r->subprocess_env variables HTTP2 and H2PUSH, set to "on"

     when available for request.

     [Stefan Eissing]

  *) mod_http2: new config directives and the implementation behind

     them: H2Timeout, H2KeepAliveTimeout, H2StreamTimeout. Documentation in

     the http2 manual.

     [Stefan Eissing]

  *) mod_http2: fixed bug in input window size calculation by moving chunked

     request body encoding into later stage of processing. Fixes PR 58825.

     [Stefan Eissing]

Changes with Apache 2.4.18

  *) mod_ssl: for all ssl_engine_vars.c lookups, fall back to master connection