indexer: Revamp walk function to use FTP MLSD
indexer: Revamp walk function to use FTP MLSD

This makes indexing a lot faster.

Requires Python 3.3:
(http://docs.python.org/dev/library/ftplib.html#ftplib.FTP.mlsd)

bbc2/porygon-ftp,bbc2/porygon-ftp,bbc2/porygon-ftp