Change Apache Geode/Pivotal GemFire 'log-level' to 'error'.

Fix compiler warnings.

Fix Javadoc warnings.

samples/javaconfig/gemfire-p2p/src/main/java/sample/Config.java

@@ -21,7 +21,7 @@ import org.springframework.data.gemfire.config.annotation.PeerCacheApplication;
 import org.springframework.session.data.gemfire.config.annotation.web.http.EnableGemFireHttpSession;
 
 // tag::class[]
-@PeerCacheApplication(name = "SpringSessionSampleJavaConfigGemFireP2p", logLevel = "warning") // <1>
+@PeerCacheApplication(name = "SpringSessionSampleJavaConfigGemFireP2p", logLevel = "error") // <1>
 @EnableGemFireHttpSession // <2>
 @EnableManager(start = true) // <3>
 public class Config {