[bug]: allow stopping of lnd during recovery rescan, properly resume after restart

[guggero]

I thought there was an issue or PR or something already concerning this problem, but I couldn't find it...

Currently lnd refuses to shut down if it is in recovery mode, because that wouldn't be properly resumed on the next start.
We should persist the rescan mode and properly resume afterwards, making this "refusal to stop" unnecessary.

Some additional context: #7105