txm: avoid excess conflict while reading gaps

During iteration a memtx tree index must write gap records to TX
manager. It is done in order to detect the further writes to that
gaps and execute some logic preventing phantom reads.

There are two cases when that gap is stores:
 * Iterator reads the next tuple, the gap is between two tuples.
 * Iterator finished reading, the gap is between the previous
tuple and the key boundary.

By a mistake these two cases were not distinguished correctly and
that led to excess conflicts.

This patch fixes it.

Part of #6206