Bump Netty version to 4.0.21.Final for some bug fixes.

Author: rxin

pom.xml

@@ -420,7 +420,7 @@
       <dependency>
         <groupId>io.netty</groupId>
         <artifactId>netty-all</artifactId>
-        <version>4.0.17.Final</version>
+        <version>4.0.21.Final</version>
       </dependency>
       <dependency>
         <groupId>org.apache.derby</groupId>