It's a regression of rb#2089: make cascade operation iterative.

trx->fts_next_doc_id is used by two cascade operations in
row_ins_cascade_calc_update_vec(), so we have duplicated
entry error. The fix is simple: allocate a doc id from
cascade heap for each operation.

Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>
RB: 8973