memtx: track read story in when conflicting full scans due to gap write

[CuriousGeorgiy]

When conflicting transactions that made full scans in memtx_tx_handle_gap_write, we need to also track that the conflicted transaction has read the inserted tuple, just like we do in gap tracking for ordered indexes — otherwise another transaction can overwrite the inserted tuple in which case no gap tracking will be handled.

Closes #7493

NO_DOC=bugfix