Prevent listeners being added multiple times on restart

(and at other times potentially - e.g. if 2 contexts share a parent).

Plus some general tidy up on compiler warnings.

Fixes gh-613

spring-cloud-context/src/test/java/org/springframework/cloud/context/restart/RestartIntegrationTests.java

@@ -59,7 +59,7 @@ public class RestartIntegrationTests {
 		then(this.context.getParent().getParent()).isNull();
 
 		RestartEndpoint next = this.context.getBean(RestartEndpoint.class);
-		then(next).isNotSameAs(endpoint);
+		then(next).isSameAs(endpoint);
 		this.context = next.doRestart();
 
 		then(this.context).isNotNull();