Persist historical channels

[benthecarman]

When a channel is closed, all metadata is lost. list_channels() only returns open channels, and would require the user to do their own after persistence after a ChannelClosed event. Applications have no way to answer basic questions after closure: how large was the channel, final balance, closing reason, funding txo, etc. This makes it difficult to do proper accounting without out-of-band tracking.

Would be good to persist channels so we have a historical record of everything we can.

[tnull]

Yeah, we probably should at some point, tagging that for 0.9.