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
* Split KopiaWrapper into repo handle and logic
With ModelStore, multiple structs need a reference to the kopia repo.
Make a small wrapper class (conn) that can open and initialize a repo. The
wrapper handles concurrent closes and opens and does ref counting to
ensure it only drops the kopia handle when the last reference is closed.
Rename KopiaWrapper to Wrapper and keep backup/restore functionality
in it.
