Bucketing is generalization of coin chooser logic that makes it easy
to implement other algorithms.
- Put core coin chooser functionality in base class.
- Specialize derived class to implement classic electrum algorithm of
oldest coins first. One bucket per output.
No intended change in behaviour.
Coin chooser now sorts the coins as it wants; remove redundant sorting
from get_spendable_coins().
9a6dcf7b1e