Track signing of local txn in channelmonitor and refuse updates

Track signing of local txn in channelmonitor and refuse updates

In e46e183084ed858f41aa304acd78503aea1a96ed we began tracking
whether a local commitment transaction had been signed and
broadcast in OnchainTxHandler, refusing to update the local
commitment transaction state in the ChannelMonitor on that basis.

This is fine, except that it doesn't make a lot of sense to store
the full local transaction state in OnchainTxHandler - we should be
providing it the unsigned local transaction at the time we wish to
broadcast and no more (just like we do all other transaction data).