txm: track read more carefully

The previous commit fixed a bug that caused dirty read but also
introduced a much less significat problem - excess conflict in
some cases.

Usually if a reader reads a tuple - in its story aspecial record
is stored. Any write that replaces or deletes that tuple can now
cause conflict of current transaction.

The problem happened when a reader tries to execute select from
some index, but only deleted story is found there. The record is
stored and that is good - we must know when somebody will insert
a tuple to this place in index. But actually we need to know it
only for the index from which the reader executed select.

This patch introduces a special index mask in read tracker that is
used in the case above to be more precise in conflict detection.

Closes #6206