Bump the 6.x branch to 6.4.0 (#29683)

This commit bumps the 6.x branch of elasticsearch to 6.4.0.

Author: jasontedor

buildSrc/version.properties

@@ -1,4 +1,4 @@
-elasticsearch     = 6.3.0
+elasticsearch     = 6.4.0
 lucene            = 7.3.0
 
 # optional dependencies

docs/Versions.asciidoc

@@ -1,4 +1,4 @@
-:version:               6.3.0
+:version:               6.4.0
 :major-version:         6.x
 :lucene_version:        7.3.0
 :lucene_version_path:   7_3_0

server/src/main/java/org/elasticsearch/Version.java

@@ -166,7 +166,9 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final Version V_6_2_5 = new Version(V_6_2_5_ID, LUCENE_7_2_1);
     public static final int V_6_3_0_ID = 6030099;
     public static final Version V_6_3_0 = new Version(V_6_3_0_ID, org.apache.lucene.util.Version.LUCENE_7_3_0);
-    public static final Version CURRENT = V_6_3_0;
+    public static final int V_6_4_0_ID = 6040099;
+    public static final Version V_6_4_0 = new Version(V_6_4_0_ID, org.apache.lucene.util.Version.LUCENE_7_3_0);
+    public static final Version CURRENT = V_6_4_0;
 
     static {
         assert CURRENT.luceneVersion.equals(org.apache.lucene.util.Version.LATEST) : "Version must be upgraded to ["
@@ -179,6 +181,8 @@ public static Version readVersion(StreamInput in) throws IOException {
 
     public static Version fromId(int id) {
         switch (id) {
+            case V_6_4_0_ID:
+                return V_6_4_0;
             case V_6_3_0_ID:
                 return V_6_3_0;
             case V_6_2_5_ID: