digest.py: use threading instead

At first I used multithreading because threading would still run with
one CPU core due to the GIL. That probably happened because I accessed
_hashes[algos] in the loop of _queue_updater. Now that this is not
done anymore, and only the hash update function is called which releases
the GIL for data larger than 2 KiB, multiple cores are actually used.

For comparison, for a file of 2.3 GiB (min/avg/max/sd secs for n=10):

 - pee sha256sum md5sum < file: 16.5/16.9/17.4/.305
 - python3 digest.py -sha256 -md5 < file: 13.7/15.0/18.7/1.77
 - python2 digest.py -sha256 -md5 < file: 13.7/15.9/18.7/1.64
 - jacksum -a sha256+md5 -F '#CHECKSUM{i} #FILENAME': 32.7/37.1/50/6.91

The file is actually 2367029248 bytes, resident in the disk cache.

Environment:
 - CPU: Intel i5-460M
 - Arch Linux x86_64
 - Linux 3.17-rc4
 - coreutils 8.23
 - moreutils 0.51
 - jacksum 1.7.0 on OpenJDK 1.7.0_71
 - Python 3.4.2, Python 2.7.8

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