When we detect a channel `is_shutdown()` or call on it
`force_shutdown()`, we notify the user with a Event::ChannelClosed
informing about the id and closure reason.
MessageSendEvent::PaymentFailureNetworkUpdate served as a hack to pass
an HTLCFailChannelUpdate from ChannelManager to NetGraphMsgHandler via
PeerManager. Instead, remove the event entirely and move the contained
data (renamed NetworkUpdate) to Event::PaymentFailed to be processed by
an event handler.
Now that NetworkGraph uses interior mutability, the RwLock used around
it in NetGraphMsgHandler is no longer needed. This allows for shared
ownership without a lock.
This doesn't exhaustively test closing fee negotiation at all, but
ensures that it is at least basically able to come to consensus and
sign cooperative closing transactions.
Because ln::functional_tests if over 9000 LoC long, its useful to
move tests into new modules as we can. Here we move all
cooperative shutdown related tests into a new module entitled
`shutdown_tests`
