For most of the past 20 years, Wal-Mart Stores Inc. has let another company build and operate gas stations in the parking lots of its stores. Now, the retailer has decided it wants to pump its own gasoline. Last week, Wal-Mart executives told Murphy USA Inc. that going forward it will build and operate its own gas stations. Murphy will continue to run the more than 1,000 locations it has already built near Wal-Mart stores. …