sweep: continue sweeping at previous feerate when reaggregating

[morehouse]

The sweeper currently aggregates HTLC claims with the same deadlines into the same sweep transaction. If the channel peer then claims one of those HTLCs on chain, the sweeper's aggregated transaction becomes invalid. The sweeper then removes the claimed HTLC and reaggregates the remaining HTLC claims into a new transaction.

When this occurs, the fee function gets reset and the feerate for the new transaction starts at the minimum again.

Instead, we should continue following the intended feerate curve after reaggregation.

Implementation

For the current linear fee function, we could make use of the existing StartingFeeRate field in SweeperInput.params to store the previous feerate value when an input is marked PublishFailed. Then on reaggregation the linear fee function will start there instead of back at the minimum.

If we ever add more complex fee functions, we'd need to do something more complicated.