Lock a task

[billyshambrook]

Currently when you get a task, it removes it from the queue. This means if the worker that picked up the task fails, it must put the task back on the queue itself.

Instead, when you get the task, it should lock it to the worker, and the worker can choose to either consume (delete) the task if it has processed it successfully, or release the lock and let another worker pick it up. If the worker dies unexpectedly, the lock should be released.