Make balance_hash less predictable

[pirapira]

balance_hash's are meant to hide concrete transferred amounts from MonitoringServices, but in the current form, they have not enough entropy so that an observer can easily recover the contents of balance hashes from monitoring requests.

This issue keeps track of adding a salt to balance_hash.

[pirapira]

A native implementation causes

data/source/raiden/TokenNetwork.sol:735:9: Compiler error: Stack too deep, try removing local variables.