Improve the StatefulSessionValve to refresh the jcr session tied to a http session only when there were changes in the repository

It is inefficient to refresh a jcr session when nothing changed in the repository: it needs to rebuild all its virtual layers again.

Currently, we cannot easily reuse the logic of the session pools which have their jcr sessions only refreshed when they did not have a refresh after the last time somerthing in the repository changed