* logging: cleaned up stderr logging
- do not show module
- do not show timestamps by default (enable with --console-timestamps)
* logging: replaced most printStderr() with log.Info
* cli: additional logging cleanup
They now uniformly support 3 flags:
--prefix=P selects contents with the specified prefix
--prefixed selects contents with ANY prefix
--non-prefixed selects non-prefixed contents
Also changed content manager iteration API to support ranges.
cli: add --prefix to 'blob gc' and 'blob stats'
* 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
