* The concept of the cleaned flag is flawed: Once we returned the connection

* The concept of the cleaned flag is flawed: Once we returned the connection
  to the pool we cannot longer rely on it as another thread could have leased
  the connection in the meantime and might have modified it.
  BUT: We only use this flag once we returned the connection to the pool.
  So signal that we returned the connection to the pool by something that is
  local to the thread, in this case set backend to NULL if we already have
  returende the connection.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1052314 13f79535-47bb-0310-9956-ffa450edef68