txm: use read trackers instead of direct conflict

Before this patch when a transaction has performed a write to
read gap (interval) a conflict record has beed created for the
reader of this gaps. That is wrong since the next writer of the
same value will not find a gap - the gap has been splitted into
parts.

This patch fixes that and create a special read tracker that was
designed specially for further tracking of writes.

This also requires writer to search for read trackers not only
in prepared stories but also in in-progress stories too.

Part of #5999