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/admin-handlers_test.go

@@ -268,7 +268,9 @@ func prepareAdminXLTestBed() (*adminXLTestBed, error) {
 	initNSLock(isDistXL)
 
 	// Init global heal state
-	initAllHealState(globalIsXL)
+	if globalIsXL {
+		globalAllHealState = initHealState()
+	}
 
 	globalConfigSys = NewConfigSys()