Change Dataset.Pull to use a single algorithm to pull data from a
source to a sink, regardless of which (if any) is local. The basic
algorithm is described in the first section of pulling.md. This
implementation is equivalent but phrased a bit differently. The
algorithm actually used is described in the second section of
pulling.md
The main changes:
- datas.Pull(), which implements the new pulling algorithm
- RefHeap, a priority queue that sorts types.Ref by ref-height and
then by ref.TargetHash()
- Add has() to both Database implementations. Cache has() checks.
- Switch Dataset to use new datas.Pull(). Currently not concurrent.
Toward #1568
Mostly, prune reachableChunks
