Move duplicates from trx_t to row_prebuilt_t

[t1meshift]

The field was placed in the wrong place, so it is relocated to row_prebuilt_t. In MDEV-17814, there is a backport of the similar fix from MySQL 8.0, but it's more complicated than mine.

[dr-m]

I think that this must be deferred until MDEV-20605 has been fixed. This is a prerequisite for MDEV-515, and could live in a MDEV-515 development branch until then.

[robertbindar]

Hey @dr-m! Is this PR still relevent for InnoDB code? It's been sitting for a while :( and lots of changes happened.
If it's still relevant, but needs to wait more till the work you mentioned above is completed, please let me know.

[dr-m]

In 8128a46 we added one more condition on the lock holder transaction lock->trx in lock_rec_inherit_to_gap(). While I do think that trx->duplicates is somewhat misplaced, I don’t think we can simply remove it. Perhaps we would need to remember in the lock itself if it had been created as part of executing a REPLACE or INSERT…ON DUPLICATE KEY UPDATE statement. That would seem to fix potential problems that we may currently have with multi-statement transactions.