VipnodePool: Support self-withdraw using operator-signed payloads

[shazow]

Right now, the operator must execute and pay for the transaction fee to allow others to withdraw (unless the ForceWithdraw flow is completed).

This requires the operator to maintain a balance in the wallet to pay for fees, which makes things logistically harder.

Would be nice if the operator could provide a signed payload for the withdraw request (approving a specific amount) that the withdrawer can use to perform their own transaction.

One gotcha: If the client requests a withdraw, the pool signs over a withdraw confirmation they can use to self-withdraw, there is no way to force the client to actually use it. That's probably okay, since it falls back to the ForceWithdraw flow.