* 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.
* add operation results structs
Operations, both backup and restore, need to hold the
results of their operation, and be able to marshal the struct
to json for output.
* introduce operations (#114)
Adds the /internal/operations package. This is a starting point
which only includes the base operation struct and handling.
Per-process (backup/restore) operations to come at a later time.
* adding comments for clarity in op_progress
