REF: Simplify quantile, remove reduction from BlockManager
BlockManager.reduction is only ever called for quantile.  Might as well remove the layer of indirection so we can simplify reduction (now renamed quantile).  Most of the simplification comes in Block.quantile, since we can avoid passing around things we don't need.

Two nested functions currently defined inside Block.quantile are moved outside the closure so I don't have to double-check the namespace every time I look at them.  Not sure if they belong somewhere else.