Add self-healing feature (#7604)

- Background Heal routine receives heal requests from a channel, either to
heal format, buckets or objects
- Daily sweeper lists all objects in all buckets, these objects
don't necessarly have read quorum so they can be removed if
these objects are unhealable
- Heal daily ops receives objects from the daily sweeper
and send them to the heal routine.

cmd/tree-walk-pool.go

@@ -34,6 +34,7 @@ type listParams struct {
 	recursive bool
 	marker    string
 	prefix    string
+	heal      bool
 }
 
 // errWalkAbort - returned by doTreeWalk() if it returns prematurely.