Not sure how this worked before but, the PPU calculation critically
requires that the order of clearing transactions are in the correct
chronological order! Fix this by sorting `trans: dict[str, Transaction]`
in the `PpTable.update_from_trans()` method.
Also, move the `get_likely_pair()` parser from the `kraken` backend here
for future use particularly when we revamp the asset-transaction
processing layer.
de655bfe6a