Fix dead letter cycle detection

[ansd]

The desired correct dead letter cycle detection behaviour is:

If there is a death with a source queue that is the same as the target
queue name and there are no newer deaths with the 'rejected' reason then
consider this a cycle.

This commit implements this desired behaviour.
Previously, we relied on the arbitrary order of keys returned from maps:keys/1 which was wrong.

[kjnilsson]

what do you think of the idea of "simply" changing the death records to a list that is kept in the modification order?

[ansd]

While this PR improves cycle detection, it's still not accurate in all cases because we rely on wall clock time.
Yes, a list defining a clear order of death events (across nodes) is therefore better.