* 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
Maintenance: support for automatic GC
Moved maintenance algorithms from 'cli' to 'repo/maintenance' package
Added support for CLI commands:
kopia gc - performs quick maintenance
kopia gc --full- perform full maintenance
Full maintenance performs snapshot gc, but it's not safe to do this automatically possibly in parallel to snapshots being taken. This will be addressed ~0.7 timeframe.
