sBTC Design - Handling Fee Cap on Withdrawal and Wallet Handoff

[AshtonStephens]

This is a research and design discussion relating to enforcement of both a handoff and withdrawal fee cap via a consensus critical mechanism AND how to handle the fee cap being below the going rate for Bitcoin transaction fees.

In order to avoid race conditions the fee cap needs to be determined every reward cycle. This presents an issue when the fee cap is below the going Bitcoin fee rate because it will be a significant amount of time before it can be updated. To handle this we present several mitigation strategies:

1. high max fee rate at the beginning of the reward cycle
2. Add SIGHASH_ANYONECANPAY when the fee cap is exceeded (stackers would need to know the fee cap is insufficient)
3. Stackers make transactions and ask for help from an explicit entity when the fee cap is reached.

The third option is more viable than it might look because any app or other entity would be reasonably willing to add more bitcoin to the withdrawal fee in order to get it out.

Resulted from an ad-hoc discussion with @jcnelson and @setzeus.