Improve IP cleanup query

[abcang]

• Changed to delete SessionActivation every 1000
• Do not update users whose last_sign_in_ip is already null
• Split select and update because update process that changes last_sign_in_ip to null is slow

before

User Update All (4.1ms)  UPDATE "users" SET "last_sign_in_ip" = NULL WHERE (last_sign_in_at < '2018-09-17 03:21:18.721436')

after:

(3.7ms)  SELECT  "users"."id" FROM "users" WHERE (last_sign_in_at < '2018-09-17 03:21:18.721436') AND "users"."last_sign_in_ip" IS NOT NULL ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1000]]
User Update All (20.1ms)  UPDATE "users" SET "last_sign_in_ip" = NULL WHERE (last_sign_in_at < '2018-09-17 03:21:18.721436') AND "users"."last_sign_in_ip" IS NOT NULL AND "users"."id" IN ($1, $2, $3, $4, $5, $6)  [["id", 7], ["id", 8], ["id", 9], ["id", 10], ["id", 13], ["id", 14]]