Improve the fix for https://issues.apache.org/bugzilla/show_bug.cgi?id=50201

Improve the fix for https://issues.apache.org/bugzilla/show_bug.cgi?id=50201
- Use AtomicReference to guard against possible duplicate registration of
  listeners when logAccess() is called by several threads at the same time.
- Added install() and uninstall() method to AccessLogListener
  to help register/unregister the listeners.
- When defaultAccessLog is nulled, always unregister the listener as well.
  The listener does not need to be reused, because now we do create a listener
  when there is no log and it will take care of things.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1045003 13f79535-47bb-0310-9956-ffa450edef68