The action to recreate activity tables while preserving existing messages
was unsafe for 2 reasons:
- if any error happened, messages could be lost
- it relied on Message.reactivate

Which this patch, any instance created after commit d881edd1 (Aug 2010) will
upgrade successfully. For older instances, make sure you have no activity left.

For cases where 'ALTER TABLE' would not work, a better way to implement repair
functionality would be:
- one action to backup all messages in ZODB
- and another to restore them
And maybe a security so that during the backup-clear-restore sequence,
activities can't be created nor processed.

If any column is added in the future, it would still be possible to write code
that fills them by inspecting messages.