Estimate transaction gas limit properly

[palango]

Currently we estimate the gas limit with the following code:

    def gaslimit(self, location='latest') -> int:
        gas_limit = self.web3.eth.getBlock(location)['gasLimit']
        return gas_limit * 8 // 10

In order to make the gas reserve work as intended and to lower the minimal amount of Ether a raiden user has to possess we should properly estimate the gas cost of transactions.

[palango]

Work on this in done in #2623

[palango]

@andrevmatos this was the issue we talked about.