FEAT: Implement iterators API

# FEAT: Implement iterators API

## Rationale

Easier to use and more natural API.

## Description

Implementation based on C++ iterators.
https://pmem.io/pmemkv/master/doxygen/classpmem_1_1kv_1_1db_1_1iterator.html

## API Changes

New Iterator class.

## Implementation details



## Meta