Add notion of n_val to index

Add the notion of `n_val` to Yokozuna indexes. This reduces coupling between KV buckets and Yokozuna indexes and fixes a bug in the query plan generation. Removing the coupling to buckets also allows for usage of indexes not yet envisioned, e.g. indexing internal events like audit logs.

This patch also makes small tweeks to the essential test to use a non-default `n_val` to make sure that works. It also uses an index name different from the bucket as that would have caught this bug many months ago.

Finally, this patch extends the HTTP Index API to handle `n_val` but changes are still required for PB as well as the official clients. See #290.