Work around LND bug 6039

Work around LND bug 6039

LND hasn't properly handled shutdown messages ever, and
force-closes any time we send one while HTLCs are still present.
The issue is tracked at
https://github.com/lightningnetwork/lnd/issues/6039 and has had
multiple patches to fix it but none so far have managed to land
upstream. The issue appears to be very low priority for the LND
team despite being marked "P1".

We're not going to bother handling this in a sensible way, instead
simply repeated the Shutdown message on repeat until morale
improves.