Improve remainder distribution logic

[pupnewfster]

Changes proposed in this pull request:

Improve remainder distribution to try and split the remainder as evenly as possible between the various destinations before falling back to sending to the first one it will fit in (#6617)

I also added some shortcuts to avoid attempting to send things and exit early if values hit zero.

[thiakil]

If we're being strict with tests, I'd expect to see one that verifies the amount sent to each endpoint (i.e. that the remainder was indeed split evenly)
Unless I've missed something, it's only testing that the full amount was sent 'somewhere'

[pupnewfster]

I will add verification about how much it sends. The main reason I didn't is that one of the destinations can only accept a part of it, so figured if all of it got accepted that technically means it got distributed as expected. But you are right, it doesn't hurt to be more explicit.