HADOOP-17955. Bump netty to the latest 4.1.68. (#3528)

Reviewed-by: Masatake Iwasaki <iwasakims@apache.org>
(cherry picked from commit cf4d2d826cad941ae3648ff9b6057856bb199fc4)

hadoop-project/pom.xml

@@ -141,7 +141,7 @@
     <htrace4.version>4.1.0-incubating</htrace4.version>
     <metrics.version>3.2.4</metrics.version>
     <netty3.version>3.10.6.Final</netty3.version>
-    <netty4.version>4.1.61.Final</netty4.version>
+    <netty4.version>4.1.68.Final</netty4.version>
     <snappy-java.version>1.1.8.2</snappy-java.version>
     <lz4-java.version>1.7.1</lz4-java.version>