Simplify RedisInterface usage when persisting Experiment alternatives

[andrehjr]

When persisting a list several calls are being made to ensure that each element gets the correct index.

Instead of dealing with that on a single MULTI call, we push all elements to a tmp list, and that gets renamed it right after, using RENAME overwriting the previous list.

Also, this is only called on new experiments, and whenever an experiment configuration has changed.

Before:

• N RPUSH calls would be made.
• N LSET calls would be made, depending on the scenario.
• 1 extra LTRIM

Now:

• Single RPUSH with all the alternatives O(n) where n is the number of elements
• 1 RENAME O(1).

Also removing one extra SISMEMBER check when calling RedisInterface#add_to_set.