Add support for automatic slicing in Reindex API

As of Elasticsearch 5.1, there is a support for automatically slicing
the reindexing task. See
https://www.elastic.co/guide/en/elasticsearch/reference/5.1/docs-reindex.html#docs-reindex-automatic-slice
for details.

This commit supports the setting for the v5 branch.

See #706

CONTRIBUTORS

@@ -68,6 +68,7 @@ Joe Buck [@four2five](https://github.com/four2five)
 John Barker [@j16r](https://github.com/j16r)
 John Goodall [@jgoodall](https://github.com/jgoodall)
 John Stanford [@jxstanford](https://github.com/jxstanford)
+Jonas Groenaas Drange [@semafor](https://github.com/semafor)
 Josh Chorlton [@jchorl](https://github.com/jchorl)
 jun [@coseyo](https://github.com/coseyo)
 Junpei Tsuji [@jun06t](https://github.com/jun06t)

client.go

@@ -26,7 +26,7 @@ import (
 
 const (
 	// Version is the current version of Elastic.
-	Version = "5.0.63"
+	Version = "5.0.64"
 
 	// DefaultURL is the default endpoint of Elasticsearch on the local machine.
 	// It is used e.g. when initializing a new Client without a specific URL.

reindex.go

@@ -20,6 +20,7 @@ type ReindexService struct {
 	waitForActiveShards string
 	waitForCompletion   *bool
 	requestsPerSecond   *int
+	slices              *int
 	body                interface{}
 	source              *ReindexSource
 	destination         *ReindexDestination
@@ -51,6 +52,12 @@ func (s *ReindexService) RequestsPerSecond(requestsPerSecond int) *ReindexServic
 	return s
 }
 
+// Slices specifies the number of slices this task should be divided into. Defaults to 1.
+func (s *ReindexService) Slices(slices int) *ReindexService {
+	s.slices = &slices
+	return s
+}
+
 // Refresh indicates whether Elasticsearch should refresh the effected indexes
 // immediately.
 func (s *ReindexService) Refresh(refresh string) *ReindexService {
@@ -179,6 +186,9 @@ func (s *ReindexService) buildURL() (string, url.Values, error) {
 	if s.requestsPerSecond != nil {
 		params.Set("requests_per_second", fmt.Sprintf("%v", *s.requestsPerSecond))
 	}
+	if s.slices != nil {
+		params.Set("slices", fmt.Sprintf("%v", *s.slices))
+	}
 	if s.waitForActiveShards != "" {
 		params.Set("wait_for_active_shards", s.waitForActiveShards)
 	}