The time complexity of previous one was too bad. With several tens of
concurrent transactions, we saw commits take minutes to complete and
the whole application looked frozen.

This new algorithm is much simpler. Instead of asking the oldest
transaction to somewhat restart (we used the "rebase" term because
the concept was similar to what git-rebase does), the storage gives
it priority and the newest is asked to relock (this request is ignored
if vote already happened, which means there was actually no deadlock).

testLocklessWriteDuringConflictResolution was initially more complex
because Transaction.written (client) ignored KeyError (which is not the
case anymore since commit 8ef1ddba).