Improve arbitrage cost calculation

[ghost]

The keeper tries to estimate the cost of sending the arbitrage transaction(s) to the network, subtracting that cost from expected profit in order to compare it with the minimum profit threshold. Unfortunately, the way it is currently implemented is very simple and basically wrong. The keeper always assumes that one arbitrage step costs 0.25 of the base currency, which may be ok if the base currency is SAI, but will be completely wrong if we set the base currency to WETH.

The calculation also completely abstracts from the current gas price in the network, or the gas price that will be used to send the arbitrage transaction. We should consider making the keeper aware of this price as well, as there are days when gas costs 1 GWei as well days when you have to use 100 GWei to get your transaction through.

Please bear in mind that ransactions may be send directly (in sequence) or via TxManager, we should figure out whether that should impact cost calculation or not.

Pls see the # TODO transaction costs are still in a fixed currency (SAI) here comment in opportunity.py.

[ghost]

I have removed the net profit calculation feature, so this issue is now redundant.