remove JobStatus, Task abstraction and 'control status' subcommand

Control status will be replaced by job-specific output at some point.

Task was not useful anymore with state machine, may reintroduce
something similar at a later point, but consider alternatives:

- opentracing.io
- embedding everything in ctx
	- activity stack would work easily
	- log entries via proxy logger.Logger object
- progress reporting should be in status reports of individial jobs

cmd/config.go

@@ -57,7 +57,7 @@ type PrunePolicy interface {
 }
 
 type PruningJob interface {
-	Pruner(task *Task, side PrunePolicySide, dryRun bool) (Pruner, error)
+	Pruner(side PrunePolicySide, dryRun bool) (Pruner, error)
 }
 
 // A type for constants describing different prune policies of a PruningJob