Once there lived a quartet, with a very solid background. Last Friday, they took the stage and thrilled a packed house at the Gold Strike Casino’s Millennium Theater in the Mississippi Delta community of Robinsonville. The Manhattan Transfer, after 30 years as one of contemporary jazz’s best vocal acts, reinforces that saying about getting better with age. Truth be told, this singing foursome of Janis Siegel, Tim Hauser, Cheryl Bentyne and Alan Paul doesn’t seem to age at all, entertaining