Merge r1830562 from trunk:

On the trunk:

mod_http2: restoring the v1.10.16 keepalive timeout behavioud of mod_http2 (to be verified).

ALSO: mod_http2: adding regular memory cleanup when transferring large response bodies. This
      reduces memory footprint and avoids memory exhaustion when transferring large files
      on 32-bit architectures. Fixes PR 62325.

Submitted by: icing
Reviewed by: icing, ylavic, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1832566 13f79535-47bb-0310-9956-ffa450edef68