Protect against crash in the `relation_possible` predicate with ambiguous relations.
It was still possible to get a KeyError when using relations that are ambiguous
at both ends, like (Executable, version_of, Program) & (Version, version_of, Project).
Closes #3010148.
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')