* Implement getting data for directory subtree
Return a slice of collections with data for a given directory subtree in
kopia. Traverse the full directory before creating a DataCollection
instead of sending items as they are found because future
implementations may cause blocking on send. This could reduce
parallelism because the code won't be able to find other directories to
traverse until the files are seen. Kopia also currently loads the entire
directory at once so there's not much benefit to streaming.
System will now continuing pulling data until completion and report all
errors at the end of the run.
* Tests for getting persisted subtree data including some error cases
2415addd05