Add missing `update_maps_on_chan_removal` call in signer restore

Add missing `update_maps_on_chan_removal` call in signer restore

When a channel is closed, we have to call
`update_maps_on_chan_removal` in the same per-peer-state lock as
the removal of the `ChannelPhase` object. We forgot to do so in
`ChannelManager::signer_unblocked` leaving dangling references to
the channel.

We also take this opportunity to include more context in the
channel-closure log in `ChannelManager::signer_unblocked` and add
documentation to `update_maps_on_chan_removal` and
`finish_close_channel` to hopefully avoid this issue in the future.