FCPInterface.handleGetTrusteesCount(): Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the code already contains the necessary synchronization.
Thus, only add a comment which explains why it is there.
IntroductionClient: Don't use database-stored objects without locking

They must be inside of the variable scope of the synchronized() so they
get GCed before rollback().

This is a workaround for the following issue:
https://bugs.freenetproject.org/view.php?id=6247
FCPInterface.handleGetIntroductionPuzzles(): Document why we don't sync