Expose internal stats of cache service

Having a cache at the right place with a good sizing is a key to good perf,
but at the moment default inmemory caches are not monitored. There are monitoring on higher level (directory) but it is hard except for the user and group directory to get an idea if they are needed or not.
We may expose guava cache stats or give a try with an ehcache impl to have a better overview of their usage.