Drop the `ChannelMonitorUpdateStatus::PermanentFailure` variant

Drop the `ChannelMonitorUpdateStatus::PermanentFailure` variant

When a `ChannelMonitorUpdate` fails to apply, it generally means
we cannot reach our storage backend. This, in general, is a
critical issue, but is often only a transient issue.

Sadly, users see the failure variant and return it on any I/O
error, resulting in channel force-closures due to transient issues.

Users don't generally expect force-closes in most cases, and
luckily with async `ChannelMonitorUpdate`s supported we don't take
any risk by "delaying" the `ChannelMonitorUpdate` indefinitely.

Thus, here we drop the `PermanentFailure` variant entirely, making
all failures instead be "the update is in progress, but won't ever
complete", which is equivalent if we do not close the channel
automatically.