Squash a number of TODOs.

- Staleness delta is no a proper function parameter and not replicated
  from package ast.

- Named type 'chunks' replaced by explicit '[]chunk' to avoid confusion.

- For the same reason, replaced 'chunkDescs' by '[]*chunkDescs'.

- Verified that math.Modf is not a speed enhancement over conversion
  (actually 5x slower).

- Renamed firstTimeField, lastTimeField into chunkFirstTime and
  chunkLastTime.

- Verified unpin() is sufficiently goroutine-safe.

- Decided not to update archivedFingerprintToTimeRange upon series
  truncation and added a rationale why.

Change-Id: I863b8d785e5ad9f71eb63e229845eacf1bed8534