revert slat clearing

weed/util/mem/slot_pool.go

@@ -42,9 +42,6 @@ func getSlotPool(size int) (*sync.Pool, bool) {
 func Allocate(size int) []byte {
 	if pool, found := getSlotPool(size); found {
 		slab := *pool.Get().(*[]byte)
-		for i, _ := range slab {
-			slab[i] = 0
-		}
 		return slab[:size]
 	}
 	return make([]byte, size)