Decorable has constructor (and soon, destructor) optimizations for shared_ptr, unqiue_ptr, and boost::optional. Wrapping the decoration in a class disables those optimizations, adding to the opCtx construction cost in the fast path. This change keeps the holder class as an interface, while making the decoration a shared_ptr, enabling the optimization.

Most of this patch is boring caller changes, as we're now passing around a holder which contains a reference to a shared_ptr, instead of a reference to a holder which is a shared_ptr.