Migration assistant fails when using legacy database identifier scheme

[jane-heller-bkg]

Some older deployments of re³gistry that probably origin in an earlier development phase are still using numeral identifiers for some of the database items used to feed the registers. This involves entries in the tables itemclass, localization, reference, register, registry, status. All references to those items in other tables are also affected of course.

Currently the migration assistant for version 2.4.x fails without a clear error message when trying to import such a database (TODO: log that here). One idea to fix the problem, is to change Re3gistry2Migration.MigrateItems s.th. it either can handle these legacy identifiers or to add a functionality to convert an old database scheme to match the current UUID assignment scheme.
A little tricky about that is, that within re³gistry UUIDs are currently derived deterministically from the according records data.