alternator test: add tests for sequence-number based iterators
alternator test: add tests for sequence-number based iterators

Alternator Streams already support the AT_SEQUENCE_NUMBER and
AFTER_SEQUENCE_NUMBER options for iterators. These options allow to replay
a stream of changes from a known position or after that known position.
However, we never had a test verifying that these features actually work
as intended, beyond just checking syntax. Having such tests is important
because recently we changed the implementation of these iterators, but
didn't have a test verifying that they still work.

So in this patch we add such tests. The tests pass (as usual, on both
Alternator and DynamoDB).

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200830115817.380075-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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