ChannelMonitor::get_funding_txo returns both the txid and scriptPK

ChannelMonitor::get_funding_txo returns both the txid and scriptPK

... instead of only the txid.

This is another instance of it not being possible to fully
re-implement SimpleManyChannelMonitor using only public methods. In
this case you couldn't properly register outpoints for monitoring
so that the funding transaction would be matched.