more data in the `box.stat.memtx.tx().mvcc.trackers` member

[TarantoolBot]

Since: 3.9
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_stat/memtx/#box-stat-memtx-tx

Now it includes the amount of memory used by the linked lists
referencing the stories deleted by statements.
Requested by @mkostoevr in tarantool/tarantool@7ddbf04.