THE GIFTAs rock musicians from Pete Townshend to Ray Davies could tell you, growing up in public is no piece of crumpet. Especially if the rest of the musicians you work with aren't growing in the same direction. While tension can bring out the best in a band, it also shortens the shelf life of the average rock & roll aggregation considerably. This, alas, was also the case with the Jam. Although the group started out as mod revivalists, leader and primary