Fix the per-account replay of Kafka events

An assistant's Kafka consumer handed each account event to a thread pool. Events for one account could then be applied out of order, such as a delete before an earlier freeze. Kafka keeps order only by offset, a record's position in its partition. The fix replays each account's events in offset order.

An assistant wrote apply_account_events(event_lines). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• event_lines: one string per event, "offset|account_id|op", in the order the workers finished. The list can be empty.
• Return "account_id=state" for every account with at least one valid event, sorted by account id in plain string order.

Rules the tests check:

• Skip a line unless all of these hold:
  • it splits on | into exactly 3 parts;
  • the offset is one or more digits from 0 to 9;
  • the account id is not empty;
  • the op is exactly create, freeze, activate or delete.
• An offset is a whole number written in decimal, so "0010" is offset 10.
• For each account, apply its events in offset order, lowest first. Events with the same offset keep the order they have in the list.
• Every account starts as absent. Each op changes the state only from certain states:
  • create: absent becomes active.
  • freeze: active becomes frozen.
  • activate: frozen becomes active.
  • delete: any state except absent becomes deleted.
• An op in any other state changes nothing, but the account still appears in the result.