Use os.scandir() in the local watcher to improve performances

We should investigate the use of os.scandir() instead of Path.iterdir() in LocalWatcher._get_children_info(). It should have better performances. See the PEP 471 for more information.
This counts with large sync trees.

I did a small test with thousands of files and os.scandir() is a winner over Path.iterdir() and Path.glob("*"). It would be good to check results when using network-shared folders.