Rewrote multiprocessing parts to only require one database connection

[Wessie]

List of changes:

• fingerprint_directory: Now uses multiprocessing.Pool.imap and updates database in callers process.
• fingerprint_file: Now uses _fingerprint_worker internally.
• _fingerprint_worker:
  1. Some slight changes to argument handling due to imap usage.
  2. 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.

These changes should remove some of the blocks @pguridi had while implementing the SQLAlchemy backend.

[worldveil]

So, the business with after_fork... it is no longer needed? (and never was?).

[Wessie]

At the very least we never called it. I was, and still am very sure it was needed (with these changes it is not needed anymore). Most likely due to lack of fingerprint_directory usage we never noticed.

[pguridi]

:clap:

[pguridi]

when this gets merged Ill update my branch. for sure my changes will be much simpler now.