Reduce redis roundtrips when loading Experiments

[andrehjr]

Experiment.find (used on every ab_test/ab_finished)

Before: 5 redis hits
After: 1 single hit pipelined

This is based on the following:

• Drop the redundant exists? roundtrip. If nothing is loaded, then the experiment does not exist.
• Pipeline the experiment loading. Instead of making N separated calls, uses Pipelining to get it from a single time.