By Lucia Mutikani WASHINGTON (Reuters) – U.S. existing home sales tumbled to a two-year low in May as prices jumped to a record high – topping the $400,000 mark for the first time – and mortgage rates increased further, pushing out entry-level buyers from the market. Despite the fourth straight monthly drop in sales and declining affordability, reported by the National Association of Realtors on Tuesday, the housing market remains fairly hot, with properties typically staying on the market for a record low 16 days. With supply still undesirably low, prices could remain elevated, though sellers…