MINOR: Ensure proper visibility of attribute accesses across threads (#21014)

I found those two cases while working in this area. In both cases, they
are read by multiple threads so they must either be volatile or
acquiring the appropriate lock. Using volatile is fine here.

Reviewers: Lianet Magrans <lmagrans@confluent.io>

Author: dajac

coordinator-common/src/main/java/org/apache/kafka/coordinator/common/runtime/CoordinatorRuntime.java

@@ -578,7 +578,7 @@ class CoordinatorContext {
         /**
          * The current state.
          */
-        CoordinatorState state;
+        volatile CoordinatorState state;
 
         /**
          * The current epoch of the coordinator. This represents

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupCoordinatorService.java

@@ -345,7 +345,7 @@ public GroupCoordinatorService build() {
      * The metadata image to extract topic id to names map.
      * This is initialised when the {@link GroupCoordinator#onNewMetadataImage(CoordinatorMetadataImage, CoordinatorMetadataDelta)} is called
      */
-    private CoordinatorMetadataImage metadataImage = null;
+    private volatile CoordinatorMetadataImage metadataImage = null;
 
     /**
      *