Seems like LND is hanging up on receiving these messages, even though
they're odd :(
So, when a peer connects, check if it supplies or wants peer backup
(even if it doesn't support both, it shouldn't hang up, and I didn't
want to separate the two paths).
And when we go to send our own, updated backup, check features before
sending.
Fixes: #6065
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-EXPERIMENTAL: `experimental-peer-storage` caused LND to hang up on us, so only send to peers which support it.
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commandline options).