Add getCacheKey() to CurrentUser and its implementations

getCacheKey() can be used when grouping objects by user in caches. The
first user of this new method is a PerThreadCache added in a later commit.

Change-Id: I00a9c1528e2fdd2219f511ba749074efe9d967b6

java/com/google/gerrit/server/IdentifiedUser.java

@@ -382,6 +382,11 @@ public class IdentifiedUser extends CurrentUser {
     return effectiveGroups;
   }
 
+  @Override
+  public Object getCacheKey() {
+    return getAccountId();
+  }
+
   public PersonIdent newRefLogIdent() {
     return newRefLogIdent(new Date(), TimeZone.getDefault());
   }