Change the recursive common table expression algorithm to use a queue instead

Change the recursive common table expression algorithm to use a queue instead
of a pair of tables.  Runs about 25% faster on the sudoku solver query.
The OP_SwapCursors opcode is no longer required.  The current implementation
uses just a fifo, but the plan is to change it into a queue that will support
ORDER BY and LIMIT in a recursive query.

FossilOrigin-Name: b2671e1133d2f1fbd36e7cd4b86d6cc7b528aa97