tweak release notes about the plugin system
optimize ExternalSorter to do be more likely to do an in-place sort
experimental implementation a partial sort instead of external sort for HashStore
make PartialSorter capable of estimating the size of each item in the stream to manage its memory usage
bugfix to PartialSorter: need to fill the buffer all the way after
measuring the first 100 items
a little doc on the PartialSort constructor
small optimization