StatisticsPage: Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the StatisticsPage already contains the necessary
synchronization.
Thus, only add a comment which explains why it is there.
Class WebInterface: Synchronize

This is a workaround for the following issue:
https://bugs.freenetproject.org/view.php?id=6247

Synchronize around usage of OwnIdentity/IntroductionPuzzle objects
because they are stored in the database and thus subject to 6247.
Clone the puzzle so we can keep using it without synchronization.