txm: introduce conflict tracker

There are situations when we have to track that if some TX is
committed then some others must be aborted due to conflict.
The common case is that one r/w TX have read some value while the
second is about to overwrite the value; if the second is committed,
the first must be aborted.
Thus we have to store many-to-many TX relations between breaker
TX and victim TX.
The patch implements that.

Part of #4897