Apply Some Java 8 Code Changes

* Make most functional interfaces as `@FunctionalInterface`
* Convert some abstract classes to `@FunctionalInterface` with `default` methods
* Apply Lambda style implementation in some places
* Remove `Function` in favor of similar in Java 8

*  Remove redundant code from `DefaultAmqpHeaderMapper` since we are already on Spring AMQP-2.0
* Add several ctors to the `ExpressionEvaluatingMessageListProcessor`
* Populate explicit `Boolean.class` `expectedType` from the `ExpressionEvaluatingReleaseStrategy`

spring-integration-scripting/src/main/java/org/springframework/integration/scripting/ScriptExecutor.java

@@ -22,15 +22,19 @@ import org.springframework.scripting.ScriptSource;
 
 /**
  * @author David Turanski
+ * @author Artem Bilan
  * @since 2.1
  */
+@FunctionalInterface
 public interface ScriptExecutor {
 
 	/**
 	 * @param scriptSource The script source.
 	 * @return The result of the execution.
 	 */
-	Object executeScript(ScriptSource scriptSource);
+	default Object executeScript(ScriptSource scriptSource) {
+		return executeScript(scriptSource, null);
+	}
 
 	/**
 	 * @param scriptSource The script source.

spring-integration-scripting/src/main/java/org/springframework/integration/scripting/ScriptVariableGenerator.java

@@ -27,6 +27,7 @@ import org.springframework.messaging.Message;
  * @author Oleg Zhurakousky
  * @since 2.0.2
  */
+@FunctionalInterface
 public interface ScriptVariableGenerator {
 
 	Map<String, Object> generateScriptVariables(Message<?> message);

spring-integration-scripting/src/main/java/org/springframework/integration/scripting/jsr223/AbstractScriptExecutor.java

@@ -60,11 +60,6 @@ public abstract class AbstractScriptExecutor implements ScriptExecutor {
 		}
 	}
 
-	@Override
-	public Object executeScript(ScriptSource scriptSource) {
-		return this.executeScript(scriptSource, null);
-	}
-
 	@Override
 	public Object executeScript(ScriptSource scriptSource, Map<String, Object> variables) {
 		Object result;