Like fine bourbon makers the Brits prefer to craft their TV shows in small batches. And when it comes to “Downton Abbey’’ the results are similarly worth savoring. The Carnival/Masterpiece producton begins its long-awaited second season on WGBH Sunday at 9 p.m. and dedicated viewers of the first set of episodes will be delighted by the continuing adventures both upstairs and down in the Earl of Grantham’s grand household.