Allow setting storage to :none for an experiment.

This will disable storing/caching subject assignments. This reduces overhead and required storage space for the experiment. 

We don't use the storage option for data analysis; we use the logging feature to create an audit trail for data analysis purposes. In our case this audit trail is pushed to Kafka.

In this case, you have to use a segmenter that will consistently segment an identifier to the same group. Also, you cannot assign or disqualify a subject by hand in this scenario. If the segmenter is heavy on CPU, this will also increase CPU usage.

Ergo: use this future with care.

@pseudomuto @peterjm @gmalette for review.