By Sedara Oluwadara   Chelsea are set to sign Senegal defender Kalidou Koulibaly from Napoli on a four-year deal.   The 31-year-old flew to London on Wednesday to undergo a medical and finalise the move, before he joins Chelsea on their United States tour.     Chelsea are also close to bringing in Manchester City centre-back Nathan Ake.   The pair would bridge the gap left by this summer's departures of Antonio Rudiger and Andreas Christensen to Real Madrid and Barcelona, respectively.   Earlier on Wednesday, Chelsea confirmed the signing of England forward Raheem Sterling from Manchester City in