Identity.mRequestURIString: Fix JavaDoc about when it is null
Identity.mRequestURI: Add missing @Deprecated

The JavaDoc already has it.
OwnIdentity: Prepare for storing FreenetURI as String

Fred purge-db4o removes the code from class FreenetURI which allows
convenient FreenetURI storage inside db4o.
Thus, WOT must stop storing FreenetURI objects and instead store them as
String.

- Add member variable mInsertURIString for storing it.
- Do not rename or remove the old mInsertURI variable: While db4o in
theory supports deleting member variables, and reading out the data of
deleted ones for migration purposes, I do not want to actually use that
code. I have never tested it, and I want to use it as a last resort, as
it might be buggy.
- Initialize the old variable with null as the constructors and all
other functions will be changed to ignore it.
- Notice: The WebOfTrust.upgradeDatabaseFormatVersion5() which is linked
by the added JavaDoc already will be added by following commits. Also,
the code to use this variable will follow as well of course.