Refactor clear message

- add new API function `elarm:clear/4`; no other API changes

     ```
     elarm:clear(Server, AlarmId, Source, ClearReason)
    ```
- Elarm sends the event for _clear_ and _manual clear_ in the same format

  ```erlang
    {elarm, Ref, {clear, alarm_id(), alarm_src(), event_id(), Reason}}
   when Reason :: ok | {manual, UserId} | source_gone | term()
   ```