refactor: 调整类修饰符.

通过调整修饰符, 降低其他不必要问题出现的概率.

src/main/java/net/lamgc/oracle/sentry/ApplicationMain.java

@@ -12,7 +12,7 @@ import org.springframework.context.annotation.Configuration;
  */
 @SpringBootApplication
 @Configuration
-public class ApplicationMain {
+class ApplicationMain {
 
     private final static Logger log = LoggerFactory.getLogger(ApplicationMain.class);
 

src/main/java/net/lamgc/oracle/sentry/ComputeInstanceManager.java

@@ -22,7 +22,7 @@ import java.util.stream.Collectors;
  * 计算实例管理器.
  * @author LamGC
  */
-public class ComputeInstanceManager {
+public final class ComputeInstanceManager {
 
     private final Map<String, ComputeInstance> instanceMap = new ConcurrentHashMap<>();
     private SshAuthIdentityProvider sshIdentityProvider;