Jefferson County Commissioners expressed optimism about the future, after formally emerging from bankruptcy Tuesday.    Commission president David Carrington said the key to blunting the effect of future sewer rate hikes is creating economic growth.    Commissioner Jimmie Stephens said he intends to work with business leaders to make that happen.  "Reduce the burden on our citizens by bringing in more business ... and let them shoulder the burden of our sewer debt," Stephens said.   Commissioners also expressed hopes that the county could attract more homeowners, now that the bankruptcy is over.   The county's bankuptcy exit plan slices the sewer debt by nearly half, but also sets up many years of rate hikes for sewer customers.