Seems like it was more useful before 263e75d,
but now it's only used for statistics which can easily be managed in a
different way. I see no other valid purposes for this list, a reference can
have some value for GC, but if DB user loses a reference to transaction that
is not closed there is not much DB can do. This improves ConcurrentView test
from

workers samples min             avg             50%             80%             90%             max
1       10      49.323µs        974.287µs       1.068978ms      1.112882ms      1.131938ms      1.131938ms
10      100     32.592µs        685.315µs       980.5µs         1.125385ms      1.137678ms      1.169789ms
100     1000    31.49µs         219.084µs       77.427µs        353.651µs       656.916µs       1.785808ms
1000    10000   30.668µs        1.639366ms      99.128µs        3.086665ms      5.031354ms      16.315849ms
10000   100000  30.818µs        40.893475ms     36.963667ms     78.650583ms     111.553136ms    302.412177ms

to

workers samples min             avg             50%             80%             90%             max
1       10      78.358µs        964.847µs       1.059159ms      1.073256ms      1.07551ms       1.07551ms
10      100     32.802µs        304.922µs       80.924µs        674.54µs        1.069298ms      1.220625ms
100     1000    30.758µs        304.541µs       64.192µs        397.094µs       1.101991ms      2.183302ms
1000    10000   30.558µs        1.05711ms       92.426µs        2.111896ms      3.317894ms      11.790014ms
10000   100000  30.548µs        10.98898ms      90.742µs        21.740659ms     33.020076ms     135.33094ms

Signed-off-by: Roman Khimov <roman@nspcc.ru>