*) core: adding a new hook and method to the API:

  *) core: adding a new hook and method to the API:
     create_secondary_connection and ap_create_secondary_connection()
     to setup connections related to a "master" one, as used in
     the HTTP/2 protocol implementation.

  *) mod_http2: using the new API calls to get rid of knowledge
     about how the core handles conn_rec specifics.
     Improvements in pollset stream handling to use less sets.
     Using atomic read/writes instead of volatiles now.
     Keeping a reserve of "transit" pools and bucket_allocs for
     use on secondary connections to avoid repeated setup/teardowns.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1899032 13f79535-47bb-0310-9956-ffa450edef68