Improve upon the previous implementation.

If the resource with the highest priority goes offline or becomes unavailable,
then the chat status of the contact must fall back to that of the resource with
the next highest priority.

In your example from #785, if the resource with priority 1 goes offline or
becomes unavailable, then in your implementation the chat status would stay at
online, although it must actually go to xa.

The solution is to update the resources attribute on the contact to not just be
an array or strings, but instead to be a map of resources to priorities and
statuses and to use that data structure.