Fix token balance check and UDC allowance minting

[ulope]

Fixes: #238

[hackaugusto]

I'm not 100% convinced on this fix.

Here is a test script. I just wanted to make sure that a subclass could use super() and the code of the super class would still use the subclass properties:

class A:
    @property
    def balance(self):
        return 1

    def mint(self):
        return self.balance

class B(A):
    def mint(self):
        return super().mint()

    @property
    def balance(self):
        return 2

a, b = A(), B()
print(a.balance, a.mint(), b.balance, b.mint())

The above will print 1 1 2 2, meaning that it does.

This happens on our code here, the call self.udc.mint(our_address) will end up using the balance define here, which is the balance of ud_token_address. If I understood this correctly, the formula self.config.token.max_funding - balance will eventually only negative values (assuming there is a scenario that does not use the complete UDC deposit).

[ulope]

super class would still use the subclass properties

If that wasn't the case OO would be pretty broken in Python ;)

With the additional changes it should now be correct (since the balance of the SP account is used to deposit into the separate Raiden nodes).

[hackaugusto]

This should fix #238

[ulope]

Codecov seems to be stuck. Since the tests are green and it's approved I'll force merge this.

[Dominik1999]

Yeah!