IntroductionClient.getPuzzles(): Return clone()s for database safety

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

The function only has 2 callers, and already claims to not require
synchronization, so its better to make the function return clone()s
instead of adding synchronization to the callers.
IntroduceIdentityPage: Synchronize

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

Synchronize around usage of the OwnIdentity object because it is stored
in the database and thus subject to 6247. Clone it so we can keep using
it across multiple functions - synchronizing across all of them wouldn't
be possible as they are being called from the unrelated outside web
interface code.