merge: Alternator streams get_records - fix threshold/record
merge: Alternator streams get_records - fix threshold/record

Merged pull request https://github.com/scylladb/scylla/pull/6969 by
Calle Wilund:

Fixes #6942
Fixes #6926
Fixes #6933

We use clustering [lo:hi) range for iterator query.
To avoid encoding inclusive/exclusive range (depending on
init/last get_records call), instead just increment
the timeuuid threshold.

Also, dynamo result always contains a "records" entry. Include one for us as well.

Also, if old (or new) image for a change set is empty, dynamo will not include
this key at all. Alternator did return an empty object. This changes it to be excluded
on empty.

  alternator::streams: Don't include empty new/old image
  alternator::streams: Always include "Records" array in get_records reponse
  alternator::streams: Incr shard iterator threshold in get_records

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla