Add Iter Pos method.

[cyharyanto]

The method will return the position of the current item within the iterator. This method will have O(log n.m) where m is the degree of the btree.

[cyharyanto]

To add more context: this Pos method is helpful in some scenarios

• to get a fast count of items matching a search criteria specifying lower and upper bound respectively,
• to calculate page numbers in paginated retrieval.

[yihuang]

can we maintain an internal counter during the iteration, would that be more efficient?