[datastores][memcached]  Check whether there is a need to log a debug message to avoid creating an array.
[datastores][common] SessionData#size doesn't need to be volatile anymore. It is accesses only in synchronized blocks
[datastores][common] Fix a bug that the session size was not updated when a page with the same id is added
[datastores][common] Fix concurrent access to SessionData.

Now it is more close to what org.apache.wicket.pageStore.DiskDataStore.SessionEntry does for DiskDataStore