Identity: Prepare for storing FreenetURI as String

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

- Add member variable mFreenetURIString for storing it.
- Do not rename or remove the old mFreenetURI 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.
Identity.Identity(): Use mRequestURIString

- Store the normalized FreenetURI to new local variable before passing
it to IdentityID.constructAndValidateFromURI() because that function
expects a FreenetURI, not a String
- Don't initialize mRequestURI anymore as it is deprecated and
initialized to null at its declaration.
Identity.getRequestURI(): Use mRequestURIString