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.

[yihuang]

To elaborate on this, the read-only mode is to avoid the mutation on the origin tree's isoid in the IsoCopy method, that makes IsoCopy to be a read-only method in this mode.

[yihuang]

The read-only mode is mainly to avoid the mutation on this field, so that we can have a fully read-only tree instance but supports IsoCopy method at the same time.

[yihuang]

An alternative approach is to simply add a new method IsoCopyReadOnly without adding a new mode, and make it callers's responsibility to avoid modify the origin tree in place. That'll make the PR less intrusive. WDYT?