Recovery redesign

[rustyrussell]

1. getlatestpeerbackup
2. recoverchannel for each channel in there.
3. make sure we don't send out NEW peer storage when we're recovering!!
4. make sure that peer storage correctly contains stub channels, once we're no longer recovering
5. make sure we always increment timestamp in peer backup!
6. Make decode decrypt and decode the channel backup file.
7. Make recover optionally take the channel backup.

[rustyrussell]

See also: #8425