LSS: LSM roundtable

Perhaps an incremental approach to stacking would make it happen sooner: pick a set of features that make stacking hard (eg 32-bit security ids) and define any lsm which does not use those features as "lightweight". Then allow stacking of any number or lightweight modules and zero or one heavyweight module. Hopefully that would allow Yama to be stacked. Over time add difficult features to expand the definition of lightweight until more modules can be stacked.