As it heads toward potentially closing on its $6.1 billion purchase of Covance next month, Burlington-based Laboratory Corp. of America is looking to sell $2.9 billion in senior notes to help fund the deal. LabCorp announced the offering Wednesday , with the notes being sold in four tranches with maturities between 2020 and 2045, and interest rates from 2.625 percent to 4.7 percent, depending on maturity date.