Extend OnchainTxHandler::generate_claim to optionally force feerate bump

Extend OnchainTxHandler::generate_claim to optionally force feerate bump

In the next commit, we plan to extend the `OnchainTxHandler` to retry
pending claims on a timer. This timer may fire with much more frequency
than incoming blocks, so we want to avoid manually bumping feerates
(currently by 25%) each time our fee estimator provides a lower feerate
than before.