- update queries to handle correctly the `before=""` case and RSM order (older to newer)
while having correct order when `max_items` without RSM is used (then order it newer to
older).
- put RSM code path in a separated method for better readability.
- check values of `max_items` and `from_index` and raise exception is they are not
positive
- when using `before` or `after`, check that the item ID used as reference exists, and
raise an exception otherwise.
- updated docstring to reflect the new behaviour.
- update pubsub cache use of `get_items` with RSM, to adapt to new behaviour.
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