The fragment reader currently unconditionally forwards its buffer to the
passed-in partition range. Even if this range is
`query::full_partition_range`, this will involve dropping any fragments
up to the first partitions tart. This causes problems for test users who
intentionally create invalid fragment streams, that don't start with a
partition-start.
Refactor the reader to not do any modifications on the stream, when
neither slice, nor partition-range was passed by the user.
93dd16fccc