Implement a recovery plugin

[rustyrussell]

We have:

• --recover
• Peer backup
• --offline
• emergency.recover
• rescan (for finding our funds)

We should implement a "recovery" plugin which handles:

• Detecting that we need recovery: when a peer sends a reestablish which is IN THE FUTURE, we currenrtly log a message and mark the channel not to go onchain. We should instead, begin a recover process.
• Trying to recover from various sources.

Stretch goals:

• Enhance peer backup to contain more information
• Mutual close for channels
• Scan for funds
• Continue channels for a while if we can / user chooses?
• Ability to do peer backup for random nodes in return for sats
• Ability for recovery plugin to use that to pay for peer backup / try it for restoration

[daywalker90]

good idea, if i could recover the sats i lost here #6045 that would be awesome!

[rustyrussell]

There is also a requirement to have a runtime recover command:

• Only works if node is unused (no channels, no funds, no addresses issued)
• Restarts with --recover=...

[cdecker]

Addressed in #6853