Identity: Add upgradeDatabaseFormatVersion5WithoutCommit()

Notice: The WebOfTrust.upgradeDatabaseFormatVersion5() which is already
linked  by the added JavaDoc will be added by following commits.
OwnIdentity.OwnIdentity(): Use mRequestURIString

The parent class' mRequestURI must not be used anymore.
OwnIdentity.startupDatabaseIntegrityTest(): Use mRequestURIString

... by using its getter.

This completes the replacement of Identity.mRequestURI with
Identity.mRequestURIString.
We still need to adapt other classes to not use FreenetURI for storage
anymore (including class OwnIdentity which is modified by this commit -
the commit does not deal with its personal mInsertURI which is still a
FreenetURI)