* mod_proxy_http2: fixed using the wrong "bucket_alloc" from the backend
   connection when sending data on the frontend one. This caused crashes
   or infinite loops in rare situations.
 * mod_proxy_http2: fixed a bug in retry/response handling that could lead
   to wrong status codes or HTTP messages send at the end of response bodies
   exceeding the announced content-length.
 * mod_proxy_http2: fix retry handling to not leak temporary errors.
   On detecting that that an existing connection was shutdown by the other
   side, a 503 response leaked even though the request was retried on a
   fresh connection.
 * mod_http2: fixed a bug that did cleanup of consumed and pending buckets in
   the wrong order when a bucket_beam was destroyed.
   [Stefan Eissing]