Unsafe deletion during iteration of ics29 channels in DisableAllChannels

[colin-axner]

Summary

see comment.

Fix:
First iterate over all channels collecting a slice of keys. Then iterate over the slice of keys and delete all the mappings

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• Not duplicate issue
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[crodriguezvega]

I think this can be closed because it's superseded by #821.

[colin-axner]

We should close it when the code gets deleted. #821 indirectly fixes this issue, although it is possible to close #821 without removing this code