[FLINK-6805] [cassandra] Shade indirect netty4 dependency

To relocate various *indirect* netty4 dep of ver 4.0.33.Final to
classpath of Flink flavor using Maven shade plugin.

This closes apache#4545.

flink-connectors/flink-connector-cassandra/pom.xml

@@ -72,6 +72,7 @@ under the License.
  <include>com.datastax.cassandra:cassandra-driver-core</include>
  <include>com.datastax.cassandra:cassandra-driver-mapping</include>
  <include>com.google.guava:guava</include>
+ <include>io.netty:netty-*</include>
  </includes>
  </artifactSet>
  <relocations>
@@ -83,6 +84,15 @@ under the License.
  <exclude>com.google.inject.**</exclude>
  </excludes>
  </relocation>
+ <!--
+ For the details of relocation pattern, refer the discussion at
+ https://github.com/apache/flink/pull/4545
+ FLINK-6805
+ -->
+ <relocation>
+ <pattern>io.netty</pattern>
+ <shadedPattern>org.apache.flink.cassandra.shaded.io.netty</shadedPattern>
+ </relocation>
  </relocations>
  </configuration>
  </execution>