Add IInstance.isValid() with default true return value

 patch by Jacek Lewandowski; reviewed by Mick Semb Wever for CASSANDRA-17214

src/main/java/org/apache/cassandra/distributed/api/IInstance.java

@@ -19,7 +19,6 @@
 package org.apache.cassandra.distributed.api;
 
 import java.net.InetSocketAddress;
-import java.util.List;
 import java.util.UUID;
 import java.util.concurrent.Executor;
 import java.util.concurrent.Future;
@@ -52,6 +51,14 @@ default Object[][] executeInternal(String query, Object... args)
 
     boolean isShutdown();
 
+    /**
+     * Returns true if we can perform any operation or retrieve information from this instance
+     */
+    default boolean isValid()
+    {
+        return true;
+    }
+
     Future<Void> shutdown();
 
     Future<Void> shutdown(boolean graceful);