INT-2265

fixed GemfireGroupStoreTests.testMultipleInstancesOfGroupStore() test to avoid intermittent failures

  refactored MessageGroupMetadata to eliminate the usage of TreeMap in favor of LinkedList for storing UUIDs of Messages

spring-integration-gemfire/src/test/java/org/springframework/integration/gemfire/store/GemfireGroupStoreTests.java

@@ -28,6 +28,7 @@ import org.junit.After;
 import org.junit.Before;
 import org.junit.Ignore;
 import org.junit.Test;
+
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 import org.springframework.data.gemfire.CacheFactoryBean;
 import org.springframework.integration.Message;