Reaching into the stages' structs for their receivers is, in hindsight,
more awkward than returning multiple values from constructors. By
returning the receiver, the caller can name the receiver whatever it
wants (as you would with any return value), and doesn't need to
reach into the struct for the field (which is super awkward in
combination with move semantics).
Refactored TVU, into stages
* blob fetch stage for blobs
* window stage for maintaining the blob window
* pulled out NCP out of the TVU so they can be separate units
TVU is now just the fetch -> window -> request and bank processing
