lib/db: Configurable block GC time (#6295)

Also retain the interval over restarts by storing last GC time in the
database. This to make sure that GC eventually happens even if the
interval is configured to a long time (say, a month).

lib/db/schemaupdater.go

@@ -451,5 +451,7 @@ func (db *schemaUpdater) updateSchema7to8(_ int) error {
 		return err
 	}
 
+	db.recordTime(blockGCTimeKey)
+
 	return t.commit()
 }