Set `counterparty_node_id` on `ChannelMonitor`s as they're updated

Set `counterparty_node_id` on `ChannelMonitor`s as they're updated

Historically, `ChannelMonitor`s had idea who their counterparty
was. This was fine, until `ChannelManager` started indexing by
peer, at which point it needed to know the counterparty when it saw
a `ChannelMonitorUpdate` complete. To address this, a "temporary"
map from channel ID to peer was added, but no upgrade path was
created for existing `ChannelMonitor`s to not rely on this map.

This commit adds such an upgrade path, setting the
`counterparty_node_id` on all `ChannelMonitor`s as they're updated,
allowing us to eventually break backwards compatibility and remove
`ChannelManager::outpoint_to_peer`.