[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

change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')