Walter Kronkite’s three-bedroom, five bathroom co-op at 870 United Nations Plaza is set to close today, adding some serious celebrity leverage and provenance to the hope that the city’s real estate market is boiling once again.  Reports New York Magazine, “Listing broker Joanna Simon, vice-president of Fox Residential Group and the renowned journalist’s longtime companion in his later years (she’s also the sister of singer Carly Simon), says the apartment was only on the market for four days, and the buyers — who aren’t in the news business …