xAODCoreAthenaPool: Use T_AthenaPoolAuxContainerCnv.
Rework xAODAuxContainerBaseCnv to derive from T_AthenaPoolAuxContainerCnv, to use the common thinning implementation.
Rework xAODAuxContainerBaseCnv to derive from T_AthenaPoolAuxContainerCnv, to use the common thinning implementation.