See changes in δftail.go for overview.

* t2+ΔFtail-concurrency: (39 commits)
  .
  .
  .
  .
  .
  .
  .
  .
  .
  .
  .
  .
  .
  .
  X zdata: Switch SliceByFileRev not to clone Zinblk
  .
  .
  .
  .
  .
  ...

Zinblk is potentially unbounded large.
Should also help further steps on concurrency.