CachingAsyncConsistentMap: evict failed get responses from cache

AtomixLeaderElector: evict failed getLeadership responses from cache immediately

Change-Id: I084abbe8f4669b98f85bc60dc6af9240630b7ce0

@@ -95,7 +95,12 @@ public class CachingAsyncConsistentMap<K, V> extends DelegatingAsyncConsistentMa
 
     @Override
     public CompletableFuture<Versioned<V>> get(K key) {
-        return cache.getUnchecked(key);
+        return cache.getUnchecked(key)
+                .whenComplete((r, e) -> {
+                    if (e != null) {
+                        cache.invalidate(key);
+                    }
+                });
     }
 
     @Override

@@ -135,7 +135,12 @@ public class AtomixLeaderElector extends AbstractResource<AtomixLeaderElector>
 
     @Override
     public CompletableFuture<Leadership> getLeadership(String topic) {
-        return cache.getUnchecked(topic);
+        return cache.getUnchecked(topic)
+                .whenComplete((r, e) -> {
+                    if (e != null) {
+                        cache.invalidate(topic);
+                    }
+                });
     }
 
     @Override