Faster directory checks

[hltbra]

I saw slows ZREM and HDEL when running large sorted sets, and that was because of the calls to empty_directory() (which calls os.listdir()).

In order to not retrieve information about all files in a given directory just to check if the directory is empty, I added the scandir package as a dependency. Both scandir.scandir and os.listdir() use the C function readdir(), however, with this pull request,scandir.scandir will only call readdir() once.

This showed a ~5% performance improvement for ZREM.