txm: track deleted stories

In order to preserve repeated reads transactional manager tracks
read of each transactions. Generally reads can be of two types -
those that have read a tuple or that have found nothing. The first
are stored in tuple story, the second - in special gap and hole
structures.

The problem was that reads that found a dirty tuple that was
invisible to this transaction (the story says that it is deleted)
was not stored neither in story nor in gap/holes.

This patch fixes that.

Part of #6206