Payout is prone to overflow

[elpiel]

In #290 we've introduced Multiplier Price bounds for payouts, this code is known that it can cause overflow at the moment.
The value of the multiplier has been restricted in JS, so this shouldn't cause any major issue right now, but it's a know issue that needs to be handled.

Return an Error instead or other solution (e.g. default value in case of overflow) to handle this issue in the long run.

https://github.com/AdExNetwork/adex-validator-stack-rust/blob/d2348ccaa057ad9ba6193d93c59c2f2eb5b70e46/sentry/src/payout.rs#L48

[elpiel]

Closing in favour of #323