`ChannelMonitor::best_block_updated` before `ChannelMonitor::transactions_confirmed` may needlessly delay broadcasting by a block

[TheBlueMatt]

We had a user who pointed out that we weren't broadcasting when we should have been after they called ChannelMonitor::best_block_updated with a block well after a CSV locktime and then called ChannelMonitor::transactions_confirmed with the transaction which we should have been spending (with a block height/hash a ways in the past). We would (I believe) have broadcasted the correct spending transaction after the next best_block_updated call, but we really shouldn't wait until then to broadcast if we get notified of an old transaction after its CSV is up.