fix longstanding memory leak from multiprocessing
fix longstanding memory leak from multiprocessing

apparently the Pool's processes were not closing properly, so the
garbage collector wouldn't clean up, resulting in large memory leaks.
after trying a lot of really complicated things the fix ended up being
ridiculously simple.

agrimaldi/metaseq,daler/metaseq,daler/metaseq,agrimaldi/metaseq,mrGeen/metaseq,daler/metaseq,mrGeen/metaseq,agrimaldi/metaseq,mrGeen/metaseq