mod_http2: fix for partial file buckets in master connection output, flushing...

mod_http2: fix for partial file buckets in master connection output, flushing of output revisited, uplift of some 2.4.x compat changes on slave filters

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1742260 13f79535-47bb-0310-9956-ffa450edef68