*) mod_http2: new implementation of h2 worker pool.

  *) mod_http2: new implementation of h2 worker pool.
     - O(1) cost at registration of connection processing producers
     - no limit on registered producers
     - join of ongoing work on unregister
     - callbacks to unlink dependencies into other h2 code
     - memory cleanup on workers deactivation (on idle timeouts)
     - idle_limit as apr_time_t instead of seconds

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1902005 13f79535-47bb-0310-9956-ffa450edef68