STOCK PURCHASE AGREEMENT BY AND AMONG MATTRESS FIRM, INC., FS EQUITY PARTNERS V, L.P., FS AFFILIATES V, L.P., CARLYLE MEZZANINE PARTNERS, L.P., BLACKROCK KELSO CAPITAL CORPORATION, AEA MEZZANINE (UNLEVERAGED) FUND LP, AEA MEZZANINE FUND LP, AEA...