Currently our coin selection algorithm always tries to minimize the number of ecash notes in the client database.

But if we wanted to support a feature where, say, people can send ecash notes p2p offline with some amount of trust that the other person won't double-spend them, then we might want to keep more coins on-hand because you can't go to the federation to make change when you're offline.

We should introduce some notion of coin selection policies that can achieve this.