Erica Marco <erica.marco@logilab.fr> [Tue, 17 Mar 2015 15:50:40 +0100] rev 10270
[migration] reorder system tables / meta-schema migration into bootstrap_migration
Also fix a bug in the backwards-compat eid_type_source method where it
returned an extid (possibly null) instead of a source name.
With these changes, migrating a database from cw 3.12 is now possible.
Closes #4962681