refactor generate-names.pl some more to try to improve reference locality in how the hash store is being used
batch HashStore sort_stream sorts to avoid taking too much disk space, while not paying too much of a penalty in performance
add a cleanup() method to ExternalSorter that can be called from other code to delete sorting temp files
delete sorter temp files when canceled inside the HashStore stream_sort spooling
tweak generate-names.pl to make prefix operations more efficient
reduce overall size of HashStore caches, and cache crc32 computations
update generate-names.pl tests for new ordering