Add a method to clear an existing session for a given user.

src/main/java/com/threerings/user/depot/DepotUserRepository.java

@@ -252,6 +252,14 @@ public class DepotUserRepository extends DepotRepository
         insert(sess);
     }
 
+    /**
+     * Clears the given user's existing session, if found.
+     */
+    public void clearSession (int userId)
+    {
+        from(SessionRecord._R).where(SessionRecord.USER_ID, userId).delete();
+    }
+
     /**
      * Validates that the supplied session key is still valid and if so, refreshes it for the
      * specified number of days.