* This is 99% mechanical:
Extracted repo.Repository interface that only exposes high-level object and manifest management methods, but not blob nor content management.
Renamed old *repo.Repository to *repo.DirectRepository
Reviewed codebase to only depend on repo.Repository as much as possible, but added way for low-level CLI commands to use DirectRepository.
* PR fixes
The hostname/username are now persisted when connecting to repository
in a local config file.
This prevents weird behavior changes when hostname is suddenly changed,
such as when moving between networks.
repo.Repository will now expose Hostname/Username properties which
are always guarnateed to be set, and are used throughout.
Removed --hostname/--username overrides when taking snapshot et.al.
Also introduced strongly typed content.ID and manifest.ID (instead of string)
This aligns identifiers across all layers of repository:
blob.ID
content.ID
object.ID
manifest.ID
