Merge of r1747531 from trunk:

mod_http2: improved cleanup of connection/streams/tasks to always
     have deterministic order regardless of event initiating it. Addresses
     reported crashes due to memory read after free issues. 



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1747532 13f79535-47bb-0310-9956-ffa450edef68