On the 2.4.x branch, merge of r1775946,1776735 from trunk:

mod_http2: adding support for MergeTrailers directive. 

mod_http2: limiting DATA frame sizes by TLS record sizes in use on the     
    connection. Flushing outgoing frames earlier. 



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1776737 13f79535-47bb-0310-9956-ffa450edef68