Better Docs/Exposed Events for current-state-behind-peer

[TheBlueMatt]

When a peer gives us a channel_reestablish message that proves we're using an old state, we give an error message and hope they broadcast, then pass a ChannelMonitorUpdateStep::ChannelForceClosed to the monitor with should_broadcast set to false. This results in a somewhat-helpful error message logged (

"Channel {} need to be shutdown but closing transactions not broadcast due to We have fallen behind - we have received proof that if we broadcast remote is going to claim our funds - we can't do any automated broadcasting" followed by
"You have a toxic holder commitment transaction avaible in channel monitor, read comment in ChannelMonitor::get_latest_holder_commitment_txn to be informed of manual action to take")

but nothing further. get_latest_holder_commitment_txn's docs could probably describe the exact case a bit better, but we should also provide an Event that more programmatically alerts the user to what happened (probably also providing a general "channel closed" Event, which we currently do not do).

Lastly, because we call force_shutdown(false) from ChannelManager::process_pending_monitor_events() when we get a CommitmentTxBroadcasted event, we'll get the second log message above in the case that our counterparty went to chain, which is bogus.