Add persistent storage with redis (bug 959248)

https://bugzilla.mozilla.org/show_bug.cgi?id=959248

* replace `save` with `then-redis` (you have to launch an instance of Redis to try)
* turns some queries' logic into map/reduce given that Redis is a key-value store
* turns all views into promise-based logic (particularly sellers)
* get rid of all `_id` introduced by `save`, now each object has a `uuid` property

Still missing:

* [ ] updating documentation (waiting for feedback)
* [ ] submitting solitude/webpay PRs (waiting for feedback)
* [ ] adding `then-redis` (waiting for readability of the PR)