feat: add api to save ChangeSet

[cool-develope]

ref: #675

To migrate the original tree to the new node-key, it needs to sync the specific version in the new format

[tac0turtle]

would you use another api to pull the change sets out and this would be writing?

[cool-develope]

would you use another api to pull the change sets out and this would be writing?

sure, since we already have the fetching api here (

iavl/immutable_tree.go

Line 337 in d2dd845

func (t *ImmutableTree) TraverseStateChanges(startVersion, endVersion int64, fn func(version int64, changeSet *ChangeSet) error) error {

)