Sync liquidation protocol with spec

[liamzebedee]

There are some aspects of the liquidation code right now that are out-of-sync / need a bit of discussion. These are:

• signer fees are currently reserved at time of minting TBTC. We don't require them to be paid in redemption anymore.
• let's not burn ETH in liquidation flows. Instead, send it to a maintainer (if possible). Implement on-chain liquidation #105 (comment)
• UX-wise, if redemption fails, it might be better to refund TBTC to the user rather than the signer bonds. Implement on-chain liquidation #105 (comment)
• consider slippage when liquidating with Uniswap (ie. if it can buy up enough TBTC) Implement on-chain liquidation #105 (comment)

[mhluongo]

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