Update balance proof in the messaging protocol

[loredanacirstea]

unique_channel_id is now defined as Channel specific data for reply attack prevention

Field Name	Field Type	Description
channel_identifier	uint	Channel identifier inside the TokenNetwork contract
token_network_address	address	Address of the TokenNetwork contract
chain_id	uint	Chain identifier as defined in EIP155

[hackaugusto]

This version is missing data for replay attack protection, one way of defining the channel id is to use a tuple:
unique_channel_id = (chain_id, token_network_adress, channel_id)
IOW, we need to add a field for the chain_id (or one block hash, but that is way harder to implement)

[loredanacirstea]

Updated description. So, instead of unique_channel_id - have 3 separate values. The smart contract needs the channel_identifier as a readable parameter (in case you guys thought of having a hash of the 3 values as the unique_channel_id).