Implement channel withdraw logic for setTotalWithdraw

[loredanacirstea]

New smart contracts now have token withdraw functionality.
Note - in the current code, the withdraw term is being used by the unlock logic. I propose to change reserve this term for the setTotalWithdraw related functionality. See #1496 for this.

TODO:

• add 2 messages: WithdrawRequest (for node requesting a withdraw signature from the partner node), Withdraw (containing the withdraw signature from the partner node)
• add corresponding events: SendWithdrawRequest, SendWithdraw, ContractSendChannelWithdraw
• add event handlers for the above
• add corresponding state changes: ActionWithdraw, ReceiveWithdrawRequest, ReceiveWithdraw, ContractReceiveChannelWithdraw (!note that both Raiden nodes need to process a node's withdraw)
• Implement new handler for the ChannelWithdraw event in blockchain_events_handler.py. Proposed name : handle_channel_(total_)withdraw

Logic sequence diagram

SC = smart contract on-chain transaction
STC = StateChange
EV = Raiden Event
MSG = Raiden Message

Feel free to improve naming.

@hackaugusto, please recheck above.

[hackaugusto]

Looks good, just two things:

• RaidenNode2 also needs to sign and handle the ContractReceiveChannelWithdraw

[hackaugusto]

Parent issue: #1383

[rakanalh]

Related issue in Raiden contracts: raiden-network/raiden-contracts#277