[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
[ticket/11691] Fix conversion test

PHPBB3-11691
Merge remote-tracking branch 'nickvergessen/ticket/11691' into develop

# By Joas Schilling
# Via Joas Schilling
* nickvergessen/ticket/11691:
  [ticket/11691] Fix conversion test
  [ticket/11691] Fix table names and arguments/docs
  [ticket/11691] Move purge code into new function
  [ticket/11691] Fix typo in comment
  [ticket/11691] Add order by to the query
  [ticket/11691] Fix some minor comments
  [ticket/11691] Fix some more problems with softdelete update
  [ticket/11691] Change detection of effectively_installed()
  [ticket/11691] Stagger user notification reconversion even more
  [ticket/11691] Stagger the convertion of soft delete updates