/buckets/a/collections/b?_limit="5" is allowed

While investigating how best to implement the Kinto side of push notifications, I settled on using ETag as the "current version" to include in push notifications. I was worried about this at first, but @leplatrem pointed out that we support `?_since="quoted Etag"`. That's interesting, because typically `"123"` should be a string rather than a number (because we parse values with JSON). But it turns out that not only do we support `?_since="123"` (which we probably should, because it matches ETags), but we also support `?_limit="123"` (which we probably shouldn't, because there's no string format for limits).

Start by reproducing the collection request `/buckets/a/collections/b?_limit="5"` and trace the API's query-parameter parsing and validation. Confirm the expected behavior for quoted `_limit` and quoted `_since`; done means quoted limits are rejected without changing the supported ETag-compatible `_since` behavior.

Written by the indexing model from the issue text.