Allow defining local indexes.

Filtering and ordering being currently achieved in-memory, the performance penalty can be important in case of large datasets.

We should allow users to create indexes on fields and rely on native IDB queries to speed things up.

Suggested API:

``` js
const db = new Kinto();
const tasks = db.collection("tasks");
tasks.ensureIndexed("label", "done");
```

Or even:

``` js
const tasks = db.collection("tasks", {
indexes: ["label", "done"]
});
```

Thoughts? @Natim @leplatrem

Start at the collection API and the current in-memory filtering and ordering path, then inspect how native IndexedDB queries are used. Decide between the suggested ensureIndexed API and collection options, and confirm that indexed fields are used for filtering and ordering on large datasets.

Written by the indexing model from the issue text.