merge_sorter: Free merger before temp_files.
pipelining/sort.h: Add comment about early initialization of merge_sorter.
Add merge_sorter::release.

Call release() after pulling to free the resources used by the merge
sorter (temporary files and sorting buffer).
pipelining/sort.h: Call merge_sorter::release in go() and end().
Fix virtual_chunk::operator bool.

Some implementations of boost::shared_ptr do not include an implicit
conversion to bool.
Merge branch 'merge_sort_release'. Fixes #123.