Also, factor out a separate NewDataStoreWithRootTracker() since
the common case is to use the same value for both the ChunkStore
and the RootTracker.
Fixes#134
In addition to putting in the 'pull' tool that I forgot to add in my initial PR,
I added an extra unit test to cover a case that we found to be buggy, as well
as addressing some comments by aa and arv.
1) Switched to io.Copy in CopyChunks
2) Added NewFlagsWithPrefix()
3) Cleaned up some error reporting
This initial implementation requires that both the "remote" and local
ChunkStores be accessible by the machine running the pull command.
I took an initial pass at splitting up the functions so that, e.g.,
calculating which refs are needed could be done on an actual remote
machine, and we can add a chunk copying routine that gets data from
the network or something.
Towards issue #81
