Change the name of the SorterThread object to "SortSubtask" to avoid confusion
with the SQLiteThread object.
Fix some problems with OOM handling in vdbesort.c.
Add a big introductory comment to vdbesort.c explaining its operation at a
high level.  Also adjust some symbolic names and fix other comment issues in
that file.
Refactor local object and method names in vdbesort.c so that their names
more closely reflect their actual use.