engine: Slightly reorganize machineTxn
engine: Slightly reorganize machineTxn

I think this makes it somewhat easier to read.  Also, it improves our
gocyclo score on goreportcard.

LuiseV/quilt,akshayknarayan/quilt,ejj/quilt-dev,secant/quilt,ejj/quilt-dev,stevenlin598/quilt,ejj/quilt-dev,quilt/quilt,kklin/kelda,PenguinToast/quilt,akshayknarayan/quilt,kklin/quilt,quilt/quilt,kelda/kelda,LuiseV/quilt,ZhongtianWang/quilt,akshayknarayan/quilt,stevenlin598/quilt,kklin/kelda,tejasmanohar/quilt,kelda/kelda,kayousterhout/quilt,tejasmanohar/quilt,tejasmanohar/quilt,PenguinToast/quilt,kklin/kelda,quilt/quilt,secant/quilt,PenguinToast/quilt,quilt/quilt,NetSys/quilt,kelda/kelda,stevenlin598/quilt,stevenlin598/quilt,LuiseV/quilt,mmussomele/quilt,kklin/quilt,secant/quilt,ZhongtianWang/quilt,PenguinToast/quilt,ZhongtianWang/quilt,NetSys/quilt,mmussomele/quilt,stevenlin598/quilt,secant/quilt,mmussomele/quilt,PenguinToast/quilt,NetSys/quilt,secant/quilt,NetSys/quilt,kayousterhout/quilt,kayousterhout/quilt,kklin/quilt,tejasmanohar/quilt,LuiseV/quilt,mmussomele/quilt,mmussomele/quilt,kelda/kelda,mmussomele/quilt,LuiseV/quilt,kayousterhout/quilt,ejj/quilt-dev,ZhongtianWang/quilt,kklin/kelda,kklin/quilt,stevenlin598/quilt,NetSys/quilt,akshayknarayan/quilt