Fix concurrency issue in Text class

After updating the Text class to use ByteBuffer in elastic#127666 we saw test
failures where similar Text instances are shared between different
threads and tested for equals(). The reason is that calling bytes()
lazily materializes the internal ByteBuffer. That method is what the
equals method calls on both instances it tests. Apparently this can
leads to race conditions when instances are shared across threads.
Making the internal `bytes` representation volatile fixes the problem.

Closes elastic#128029

Author: cbuescher

libs/x-content/src/main/java/org/elasticsearch/xcontent/Text.java

@@ -31,7 +31,7 @@ public static Text[] convertFromStringArray(String[] strings) {
         return texts;
     }
 
-    private ByteBuffer bytes;
+    private volatile ByteBuffer bytes;
     private String text;
     private int hash;
     private int stringLength = -1;