This adds a `state_change` 'cause' to a channel.
A 'cause' is some initial 'reason' a channel was created or closed by:
/* Anything other than the reasons below. Should not happen. */
REASON_UNKNOWN,
/* Unconscious internal reasons, e.g. dev fail of a channel. */
REASON_LOCAL,
/* The operator or a plugin opened or closed a channel by intention. */
REASON_USER,
/* The remote closed or funded a channel with us by intention. */
REASON_REMOTE,
/* E.g. We need to close a channel because of bad signatures and such. */
REASON_PROTOCOL,
/* A channel was closed onchain, while we were offline. */
/* Note: This is very likely a conscious remote decision. */
REASON_ONCHAIN
If a 'cause' is known and a subsequent state change is made with
`REASON_UNKNOWN` the preceding cause will be used as reason, since a lot
(all `REASON_UNKNOWN`) state changes are a subsequent consequences of a prior
cause: local, user, remote, protocol or onchain.
Changelog-Added: Plugins: Channel closure resaon/cause to channel_state_changed notification
8a8dabaa58