feat(config): Allow sending track_total_hits with ES query

A few versions ago, Elasticsearch introduced an optimization where they
only approximate the total hit count for a query (max of 10,000).

It's of course nice to get better performance, but having hit count
information is also super useful to examine queries for our own
optimization.

This change allows us to enable the [`track_total_hits`](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-search#operation-search-track_total_hits) flag on ES
queries, with the similar `api.track_total_hits` boolean in
`pelias.json`.

Author: orangejulius

controller/search.js

@@ -59,6 +59,10 @@ function setup( peliasConfig, esclient, query, should_execute ){
       cmd.explain = true;
     }
 
+    if (_.get(apiConfig, 'trackTotalHits') === true) {
+      cmd.track_total_hits = true;
+    }
+
     // support for the 'clean.exposeInternalDebugTools' config flag
     let debugUrl;
     if (_.get(req, 'clean.exposeInternalDebugTools') === true) {