Use a slice to back the data instead of adding directly to the channel
for two reasons (this may change in the future though):
* kopia loads all data about a directory at the same time
* consumers of the DataCollection may not pull items from the channel
at a fast rate, which could block adding to the channel. This could
lead to delays in discovering other directories to traverse in
multi-threaded scenarios
* Change DataCollection to return channel directly
Precursor to restoring multiple items from kopia. Allows one to keep a
DataCollection open until all items are processed without blocking
consumers of the DataCollection (they can use a select-block if needed).
* Update tests for new DataCollection interface
* Handle context cancellation with DataCollection
* DataCollection and DataStream structs for kopia
* Helper function to get a single item of data
Returns a DataCollection with a single item.
* Test for backing up and restoring a single item
