Reduce Redis roundtrips on the ab_test path

[andrehjr]

Cuts the number of Redis calls made during Split::Helper#ab_test

Changes

• Drop the redundant exists? roundtrip in Experiment.find and Experiment#save
• Batch cleanup_old_experiments! into a single pipelined roundtrip instead of two hgets per experiment (was O(N) in the user's experiment count)
• Batch active_experiments winner lookups into one hmget instead of one hget per experiment
• Add RSpec matcher to pin the roundtrip counts and guard against regressions