We now do data filtering/massage as late as possible, not just once at
startup. This should work better for multiple commands, as with web or ui.
The basic benchmark seems at least as good as before thanks to laziness.
With this change, Transactions and Postings reference each other
co-recursively. This makes constructing them more tedious, but it
may also allow LedgerPostings to be dropped and code to be simplified.
Time and space performance of register and balance is as before.
