It should be acceptable to call sendMessageToAllWorkers() more than once

Currently if you sendMessageToAllWorkers(), it assumes only a single all-workers
message is sent at a time. If you send another all-workers message before a
given worker has recieved the last all-workers message, the last all-workers
message will get lost for that worker.

This change queues up these all-workers messages instead of overwriting the last one.
2.4.1