In a 4-quart kettle cook onion in butter over moderate heat, stirring occasionally, until softened. Add corn and sugar and cook, stirring occasionally, for 3 minutes. Add broth and simmer, stirring occasionally for 5 minutes, or until corn is just tender. Add salsa and simmer, covered for 5 minutes. Add the remaining ingredients and cook, uncovered, over moderately high heat until just heated through. Season chowder with salt and pepper and serve with lime wedges.