Talk about confidence in Sixth Avenue! While some Midtown restaurateurs claim they’re struggling, Del Frisco’s Double Eagle Steakhouse — a three-level, 25,000 square-foot jumbo at 1221 Sixth Ave. — just renewed its lease with long-time landlord Rockefeller Group for 20 more years. The 2.6 million square-feet office tower above the steakhouse is more than 99 percent leased while...