* mechanical rename of package snapshot/gc => snapshot/snapshotgc
* maintenance: record maintenance run times and statuses
Also stopped dropping deleted contents during quick maintenance, since
doing this safely requires coordinating with snapshot GC which is
part of full maintenance.
* cli: 'maintenance info' outputs maintenance run history
* maintenance: only drop index entries when it's safe to do so
This is based on the timestamp of previous successful GC that's old
enough to resolve all race conditions between snapshot creation and GC.
* maintenance: added internal flush to RewriteContents() to better measure its time
* 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
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
