[oba] Pulling actual balances from the ethereum chain

[josojo]

In order to solve the problem properly, we have to investigate the balance and allowance of each owner of an order before we start solving the batch. Hence, we need some service to get us the latest information. There are different approaches:

• Reading all Transfer and Allowance events and store them on our instances: While it might be a good technical solution, its effort is quite high.
• Asking our nodes directly via smart contract reads: This might not be performant enough for many orders. Though, we don't expect too many orders to be matched at the beginning
• Asking an external API: https://etherscan.io/apis#tokens: This one will not be available for our testnets :(