fix(settlement): adds sanity check at end of settlement

[Alexangelj]

MUST MERGE chore/fmt FIRST.

Description

• Closes Investigate: Tokens with fee on transfer, unsupported #246 by adding a sanity check at the end of the _settlement function to make sure the net balance of every token in the warm array has a positive value.

This is not the most efficient function because it does three loops: loop over to get payments, loop over to do payments, loop over all tokens to make sure they were settled.

This can be all done in a single loop, however, this introduces a possible attack vector via a malicious token that executes arbitrary logic in the transferFrom which manipulates the settlement status of some of the other tokens. In a single loop, the check for the net balance would happen in between transferFrom. If there was a way to take tokens out of the contract after they have been transferred in, then there would be a way to get a negative net balance even after the sanity check after the transfer from. That is pretty unlikely though.