Middleware steps should support a slotting.

[skmcgrail]

Middleware steps should allow for a slot to be added to a given step via an AddSlot and InsertSlot API. A slot would be a named position with the middleware step that would allow other middlewares to be inserted relative to it. If a middleware is registered with a matching slot name, that middleware would fill in the slot. If a middleware is later removed, the slot reference would still remain.

Slots that are not directly assigned a middleware would have no affect on the fully constructed handler stack.