Subclass Sorter as to not require always going to file/double writing.
Comment derived class
Bump dependency
Only MergeJoinSorter closes
Set list to null after deleting
merge nathanlws: Subclass Sorter to provide a pull based interface

Merge pull request #947 from nathanlws/sql-layer/pull-based-sorter
https://github.com/FoundationDB/sql-layer/pull/947