Support users CPFP-bumping a coop close tx

[TheBlueMatt]

Currently, when we sign and broadcast a coop-close tx we do so to a user-provided script, so theoretically they could CPFP it. However, we don't tell them anything about the transaction until after its confirmed (as a SpendableOutput) so they'd have to monitor the mempool (or monitor the tx broadcaster) to do so. Ideally we'd have some way of letting users CPFP without monitoring the mempool, but its not entirely clear to me how to do it.