Merge of 1761479,1761548,1762703,1763158 from trunk

mod_http2: rewrite of how responses and trailers are transferred between
     master and slave connection. Reduction of internal states for tasks
     and streams, stability. Heuristic id generation for slave connections
     to better keep promise of connection ids unique at given point int time.
     Fix for mod_cgid interop in high load situtations. 
     Fix for handling of incoming trailers when no request body is sent.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1763163 13f79535-47bb-0310-9956-ffa450edef68