[SPARK-8101] [CORE] Upgrade netty to avoid memory leak accord to netty #3837 issues

Update to Netty 4.0.28-Final

Author: Sean Owen <sowen@cloudera.com>

Closes #6701 from srowen/SPARK-8101 and squashes the following commits:

f3b6369 [Sean Owen] Update to Netty 4.0.28-Final

pom.xml

@@ -587,7 +587,7 @@
       <dependency>
         <groupId>io.netty</groupId>
         <artifactId>netty-all</artifactId>
-        <version>4.0.23.Final</version>
+        <version>4.0.28.Final</version>
       </dependency>
       <dependency>
         <groupId>org.apache.derby</groupId>