Increase default heap size to 2g

Benchmarking with the geonames data set shows that Elasticsearch truly
needs 2g of heap or the heap is a bottleneck for indexing rates. If our
goal is to satisfy the out-of-the-box-experience, we should prioritize
the out-of-the-box performance experience. Thus, the default heap should
be 2g until smaller heaps are not the bottleneck for indexing small data
sets.

distribution/build.gradle

@@ -442,7 +442,7 @@ task run(type: RunTask) {}
  * </dl>
  */
 Map<String, String> expansionsForDistribution(distributionType) {
-  String heapSize = '512m'
+  String heapSize = '2g'
   String footer = "# Built for ${project.name}-${project.version} " +
       "(${distributionType})"
   Map<String, Object> expansions = [