!str #15271 make transform() take a factory instead of Transformer

+ Makes reusing flows safe 
+ Adds timerTransform for ops which need TimerTransformer,
  for now it was the easier way out we should rethink if we need to
  specialise a lot on it or not.

// Also, making the API more verbose. Refer to #15271 on why we want the explicit `() =>`

As discussed in the team, this is the "simple" version, no classtag magic involved (which would leak into user api).
yeah, here "chunkTransformer" would be nicer. I'll actually update these names now (did so in some places already, not everywhere).