MigrateActors: Improve query for log_search rows

MigrateActors: Improve query for log_search rows

The query in question is, generically,

 SELECT ls_value, ls_log_id, actor_id
  FROM log_search LEFT JOIN actor ON (ls_value = actor_user)
  WHERE ls_field = 'target_author_id'
  ORDER BY ls_value, ls_log_id
  LIMIT 100;

The intention is that it'll pull out 100 rows from log_search using its
primary key, and for each of those 100 rows find the actor_id for the
referenced user_id (using the actor_user index) if an actor ID exists.

The twist comes in the fact that ls_value is a string-type column while
actor_user is an integer-type. MySQL doesn't usually care, but other DBs
do so we have to cast one into the other.

Currently the code is casting actor_user to a string. But that means the
DB can't use the index on actor_user to find the one matching row,
instead it needs to scan the whole table.

The fix is simple enough: instead of casting actor_user to a string,
cast ls_value to an integer. That allows the actor_user index to be used
as expected.

Bug: T215525
Change-Id: I2f7a6ba9fd537336594088a0281a62ea5601cd59