Proper CF tx size calculation for refills

[darosior]

https://github.com/revault/watchtower_paper/blob/b616321d4a0f28d96a66d1f94d3ac123600323d5/model/simulation.py#L160-L167

[darosior]

Marking as bug as we really can't assume this in Real Life

[darosior]

I have something for this. I think the refill amount (and the decision to not refill) wasn't calculated correctly.

[darosior]

It wasn't (see #76), but this doesn't fix the issue entirely