heal: Add info about the next background healing round (#9122)

- avoid setting last heal activity when starting self-healing

This can be confusing to users thinking that the self healing
cycle was already performed.

- add info about the next background healing round

pkg/madmin/heal-commands.go

@@ -274,6 +274,7 @@ func (adm *AdminClient) Heal(bucket, prefix string, healOpts HealOpts,
 type BgHealState struct {
 	ScannedItemsCount int64
 	LastHealActivity  time.Time
+	NextHealRound     time.Time
 }
 
 // BackgroundHealStatus returns the background heal status of the