Specify properties to send back in results about api

[dianashk]

This should be taken into consideration during API redesign.

Related to pelias/pelias#42

[missinglink]

There has been a fair bit of discussion around the consumer having more control over the properties returned by API, most notably #101

There is certainly a tradeoff in trimming the db response in javascript as mentioned in #122 (comment)

If we decide to offer this level of configuration we should do it at query-time rather than in the controller code by adding a fields params to all queries. This will give us a speed-up all the way down the stack and all the way back up again, plus the problems of selectively mapping the data to the JSON response go away.

In order to accomplish this, we would need to rejig our queries a bit as the response body from elasticsearch for a fields query is different from a normal search query (._source becomes .fields)

It's also important for the API redesign discussion that we discuss what the "default params" will be, as a new user of the API you should be able to get a decent set of params that cover most usecases, this functionality would just be for super-users who want to squeeze every last bit of performance out of the service, mostly those targeting mobile phones.

[dianashk]

Hold off until API-redesign

[dianashk]

No longer relevant. Covered by the introduction of the view parameter.