Calculate the effective value of buckets, and filter <0 out.
Note that the filtering is done on the buckets, not per-coin.
This should better preserve the user's privacy in certain cases.
When the user "sends Max", as before, all UTXOs are selected,
even if they are not economical to spend.
see #5433
existing code was n^2 in number of UTXOs
this is now mostly linear
(linear if shortcut is hit; otherwise in rare cases still quadratic)
tested using wallet with 800 UTXOs, most of which were needed to make payment
coinchooser.make_tx() went from 18 sec to 0.8 sec
