FCPInterface.handleAddIdentity(): Synchronize

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

Synchronize around usage of the Identity object because it is stored
in the database and thus subject to 6247.
FCPInterface.handleGetIdentity(): Document why we synchronize
FCPInterface.handleGetOwnIdentities(): Document why we synchronize