Need more synchronizing in DruidDataSource.

server/src/main/java/io/druid/client/DruidDataSource.java

@@ -114,14 +114,17 @@ public class DruidDataSource
   @Override
   public String toString()
   {
+    synchronized (lock) {
       return "DruidDataSource{" +
              "properties=" + properties +
              ", partitions=" + segmentsHolder.toString() +
              '}';
     }
+  }
 
   public ImmutableDruidDataSource toImmutableDruidDataSource()
   {
+    synchronized (lock) {
       return new ImmutableDruidDataSource(
           name,
           ImmutableMap.copyOf(properties),
@@ -130,3 +133,4 @@ public class DruidDataSource
       );
     }
   }
+}