Revert "Remove unused CurrentUser#cacheKey method"

Revert "Adjust to changes in Gerrit core"

Revert submission 283559-currentuser-remove-cache-key

Reason for revert: Causes a latency regression for some hosts
Reverted Changes:
I76bfd3ebc:Adjust to changes in Gerrit core
If7ccfd9a4:Remove unused CurrentUser#cacheKey method
I1378ad083:Remove PerThreadCache

Change-Id: Idd9eb9cb91bb531ee75aa2c2cde26a01ceb5ff0a

javatests/com/google/gerrit/server/extensions/webui/UiActionsTest.java

@@ -100,6 +100,11 @@ public class UiActionsTest {
           throw new UnsupportedOperationException("not implemented");
         }
 
+        @Override
+        public Object getCacheKey() {
+          return new Object();
+        }
+
         @Override
         public boolean isIdentifiedUser() {
           return true;

javatests/com/google/gerrit/server/permissions/RefControlTest.java

@@ -1258,6 +1258,11 @@ public class RefControlTest {
       return groups;
     }
 
+    @Override
+    public Object getCacheKey() {
+      return new Object();
+    }
+
     @Override
     public Optional<String> getUserName() {
       return Optional.ofNullable(username);