Fixed php-cs-fixer bc-break: ALL_LEVEL to SYMFONY_LEVEL.
CS Fix: casting requires space.
Added missing php docs.
Avoid call_user_func, by passing `$this`.
Force MessageInterface for onWorkerHandlerMessage.
Refactored heartbeats within WorkerCommunication.

- Removed undefined constant STATUS_OK. (unreachable code).
- Extracted handleHeartbeatResponse.
- Grouped handling of READY, RESULT and REGISTER.