Why do we have a list instead of mappings for remembering service providers

[pirapira]

We are using a list to remember service providers and @LefterisJP says it's weird. I also think lists require more care (like never to iterate over them, lest you hit the gas limit, etc).

https://github.com/raiden-network/raiden-contracts/blob/59631b6c8b7bcb0b9a3accdf1fb41082c29dcaa1/raiden_contracts/data/source/services/ServiceRegistry.sol#L12