Clear out ClientCache when UI is detached to prevent a minor memory leak (#12200)

Implemented with a listener rather than direct call from UI.detach() in
order to avoid new public API, since the whole feature has been marked
for removal. This doesn't yet prevent the cache or the type map from
getting slightly bloated during the UI's lifetime.

See: #3705

Cherry-picked from: #12199

server/src/main/java/com/vaadin/server/LegacyCommunicationManager.java

@@ -29,6 +29,8 @@ import java.util.logging.Level;
 import java.util.logging.Logger;
 
 import com.vaadin.server.ClientConnector.ConnectorErrorEvent;
+import com.vaadin.server.ClientConnector.DetachEvent;
+import com.vaadin.server.ClientConnector.DetachListener;
 import com.vaadin.shared.ApplicationConstants;
 import com.vaadin.shared.JavaScriptConnectorState;
 import com.vaadin.shared.communication.SharedState;
@@ -189,16 +191,36 @@ public class LegacyCommunicationManager implements Serializable {
      * @deprecated As of 7.1. See #11410.
      */
     @Deprecated
-    public ClientCache getClientCache(UI uI) {
+    public ClientCache getClientCache(final UI uI) {
         Integer uiId = Integer.valueOf(uI.getUIId());
         ClientCache cache = uiToClientCache.get(uiId);
         if (cache == null) {
             cache = new ClientCache();
             uiToClientCache.put(uiId, cache);
+            uI.addDetachListener(new DetachListener() {
+                @Override
+                public void detach(DetachEvent event) {
+                    removeClientCache(uI);
+                }
+            });
         }
         return cache;
     }
 
+    /**
+     * Clear out client cache for the given UI. This should be called when the
+     * UI is detached and the cache becomes obsolete.
+     *
+     * @param uI
+     *            the UI whose client cache should be removed
+     * @deprecated because this cleanup is only needed for a deprecated feature
+     */
+    @Deprecated
+    private void removeClientCache(UI uI) {
+        Integer uiId = Integer.valueOf(uI.getUIId());
+        uiToClientCache.remove(uiId);
+    }
+
     /**
      * Checks if the connector is visible in context. For Components,
      * {@link #isComponentVisibleToClient(Component)} is used. For other types