Investigate E-S document routing

This is not essential, but will be a performance boost, especially for large indices (thinking swift).

Elasticsearch by default assigns a document to a shard (lucene instance) based on the document id, as defined in https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-routing-field.html. This means a search has to hit all shards. An easy performance gain in certain situations is to define the routing oneself; in our case, tenant_id provides a natural routing hint because the majority of non-admin queries restrict on tenant-id.

Some further thought is required, including, i think, a short spec proposal, because there are some edge cases around things like glance membership where this routing would not be reliable, but i think we could apply it in many cases.