Add randomness to worker when pulling new task

When running multiple workers, they are all notify at the same time when a new task is added to the queue. So they all starts pulling new task at the same time and this can generate errors like #11937 (closed). Indeed it will be better to have some randomness in each worker process so they do not pull at the same time. And as this is asynchronous tasks, having a small delay in the run of the task is not a big deal if it is to save some resources (like retrying pull a second time).