Experimental multi-threaded sorting changes to allow the sorter to begin returning items to the VDBE before all data is sorted.
Avoid having the sorter merge too many PMAs at a time when incrementally merging data following a SorterRewind().
Fix many issues with new code.
Improve use of multiple threads in sqlite3VdbeSorterRewind().
Minor fixes so that builds with SQLITE_MAX_WORKER_THREADS=0 work.
Improve performance in single-threaded mode by having the final merge pass keys directly to the VDBE, instead of going via a final PMA.
Allow the sorter to begin returning data to the VDBE as soon as it is available, instead of waiting until all keys have been sorted.