@FunctionalInterface added to task listeners

redisson/src/main/java/org/redisson/api/executor/TaskFailureListener.java

@@ -21,6 +21,7 @@ package org.redisson.api.executor;
  * @author Nikita Koksharov
  *
  */
+@FunctionalInterface
 public interface TaskFailureListener extends TaskListener {
 
     /**

redisson/src/main/java/org/redisson/api/executor/TaskFinishedListener.java

@@ -21,6 +21,7 @@ package org.redisson.api.executor;
  * @author Nikita Koksharov
  *
  */
+@FunctionalInterface
 public interface TaskFinishedListener extends TaskListener {
 
     /**

redisson/src/main/java/org/redisson/api/executor/TaskStartedListener.java

@@ -21,6 +21,7 @@ package org.redisson.api.executor;
  * @author Nikita Koksharov
  *
  */
+@FunctionalInterface
 public interface TaskStartedListener extends TaskListener {
 
     /**

redisson/src/main/java/org/redisson/api/executor/TaskSuccessListener.java

@@ -21,6 +21,7 @@ package org.redisson.api.executor;
  * @author Nikita Koksharov
  *
  */
+@FunctionalInterface
 public interface TaskSuccessListener extends TaskListener {
 
     /**