Merge of 1764243,1765318 from trunk:

mod_http2/mod_proxy_http2: 100-continue implementation, PING checks on aged backend connections


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1765327 13f79535-47bb-0310-9956-ffa450edef68