add ReadOnly mode and change IsoCopy behavior

[yihuang]

Context

I'm experimenting with the optimistic concurrency control pattern using the IsoCopy feature, it should be useful in read-heavy scenarios, this patch is some necessary changes to enable this pattern.

type BTree[T any] struct {
	atomic.Pointer[btree.BTreeG[T]]
}

func NewBTree[T any](less func(a, b T) bool) *BTree[T] {
	tree := btree.NewBTreeGOptions[T](less, btree.Options{
		NoLocks:  true,
		ReadOnly: true,
	})
	t := &BTree[T]{}
	t.Store(tree)
	return t
}

func (bt *BTree[T]) Get(item T) (result T, ok bool) {
	return bt.Load().Get(item)
}

func (bt *BTree[T]) Set(item T) (prev T, ok bool) {
	for {
		t := bt.Load()
		c := t.Copy()
		prev, ok = c.Set(item)
		c.Freeze()
		if bt.CompareAndSwap(t, c) {
			return
		}
	}
}

The performance difference of different concurrency control strategies differs a lot based on the workload, for my local testing scenario, the CAS is faster than Mutex in some cases, but slower in some other extreme cases, but both are much faster than RWMutex.