Add support for the sqlite3_vtab_distinct() interface. Virtual table

Add support for the sqlite3_vtab_distinct() interface.  Virtual table
implementations can use this API to determine more detail about the ordering
requirements needed by the query plan and perhaps reduce the amount of
work required to compute a correct answer.  This is an optimization
opportunity for the virtual table implementation.  The correct answer should
still be obtained (though perhaps more slowly) even if sqlite3_vtab_distinct()
is ignored.

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