AMENDED AND RESTATED OMNIBUS AGREEMENT by and among SEMGROUP, L.P., SEMMANAGEMENT, L.L.C., SEMMATERIALS, L.P., SEMGROUP ENERGY PARTNERS, L.P., SEMGROUP ENERGY PARTNERS G.P., L.L.C. and SEMMATERIALS ENERGY PARTNERS, L.L.C. dated as of February 20, 2008