Passive note management

[daira]

@nuttycom:

There's a very straightforward "passive note management" approach that we could take, which is to inspect the distribution of unspent notes prior to deciding upon the change note distribution. [...]
And since we use the change system for shielding, we could use the same logic for shielding. I think that would capture a significant portion of the use cases.
With those in place, you'd have it so that if you initially funded your wallet by sending to its transparent address, your experience would generally be pretty good; if you initially funded your wallet by sending to one of its shielded receivers, your UX would improve after your first spend and would then remain consistently better thereafter.

[nuttycom]

Completed to an initially satisfactory degree in #1580