Various caching improvements:
- Collect cache stats and make them available over JMX.
- Separate the root resources and Cache in JMX.
- Clear the cache when it is disabled.
- Clear the traced resources when tracing is disabled.
- Use a concurrent set for the traced resources.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1550397 13f79535-47bb-0310-9956-ffa450edef68