[ticket/11691] Stagger the convertion of soft delete updates

PHPBB3-11691
[ticket/11691] Stagger user notification reconversion even more

PHPBB3-11691
[ticket/11691] Change detection of effectively_installed()

When start is set for a migration, it is not yet effectively installed.
So we just continue doing it...

PHPBB3-11691
[ticket/11691] Fix some more problems with softdelete update

* wrong order of arguments on sql_query_limit
* avoid "BIGINT UNSIGNED value is out of range" errors
* increase limit for the loop

PHPBB3-11691
[ticket/11691] Fix some minor comments

PHPBB3-11691
[ticket/11691] Add order by to the query

PHPBB3-11691
[ticket/11691] Fix typo in comment

PHPBB3-11691
[ticket/11691] Move purge code into new function

PHPBB3-11691
[ticket/11691] Fix table names and arguments/docs

PHPBB3-11691