Rewrote multiprocessing parts to only require one database connection

List of changes in detail:
- fingerprint_directory:
	Now uses mp.Pool.imap and updates database in callers
	process.
- fingerprint_file:
	Now uses _fingerprint_worker internally.
- _fingerprint_worker:
	Some slight changes to argument handling due to `imap`
	usage.

	Changed to return (song_name, hashes) where hashes is
	a set of hashes from all channels.
- path_to_songname:
	Changed to use `os.path.splitext`, this caused files with
	a period in their name to return the wrong name.