Avoid Task Duplication

[nsteinberg-r7]

Following our discussion in the PR,

1. Can there be an implementation that serializes tasks identically (without time parameter)?
2. lists outperform zset that's true, but we use zset for the delayed feature already. So I must conclude that a worker that has a known limit of tasks (let's say number of assets) working with zset shouldn't worry me.
3. If only the delayed feature worked with nx=False flag, I would have "forced" the delayed feature to my needs. Like scheduling all my tasks as "delayed" with zadd. Any chance to settle for that? (receiving nx as parameter)