HBASE-19610 Fixed Checkstyle errors in hbase-protocol and enabled Checkstyle to fail on violations

hbase-protocol/pom.xml

@@ -112,6 +112,22 @@
             </replacements>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-checkstyle-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>checkstyle</id>
+            <phase>validate</phase>
+            <goals>
+              <goal>check</goal>
+            </goals>
+            <configuration>
+              <failOnViolation>true</failOnViolation>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
     <pluginManagement>
       <plugins>

hbase-protocol/src/main/java/com/google/protobuf/HBaseZeroCopyByteString.java

@@ -61,7 +61,7 @@ public final class HBaseZeroCopyByteString extends LiteralByteString {
   /**
    * Extracts the byte array from the given {@link ByteString} without copy.
    * @param buf A buffer from which to extract the array.  This buffer must be
-   * actually an instance of a {@code LiteralByteString}.
+   *            actually an instance of a {@code LiteralByteString}.
    * @return byte[] representation
    */
   public static byte[] zeroCopyGetBytes(final ByteString buf) {

hbase-protocol/src/main/java/org/apache/hadoop/hbase/util/ByteStringer.java

@@ -17,18 +17,18 @@
  */
 package org.apache.hadoop.hbase.util;
 
+import com.google.protobuf.ByteString;
+import com.google.protobuf.HBaseZeroCopyByteString;
+
 import org.apache.yetus.audience.InterfaceAudience;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import com.google.protobuf.ByteString;
-import com.google.protobuf.HBaseZeroCopyByteString;
-
 /**
  * Hack to workaround HBASE-10304 issue that keeps bubbling up when a mapreduce context.
  */
 @InterfaceAudience.Private
-public class ByteStringer {
+public final class ByteStringer {
   private static final Logger LOG = LoggerFactory.getLogger(ByteStringer.class);
 
   /**