Return a list of all reservations by subnet ID - #313 for Cassandra back-end (if possible)

This is a follow-on request from GL #313 (closed) - which was not implemented for Cassandra database back-end due to the technical challenges, described as:

In #313 (closed), the idea about paging is to reduce the communication:

with SQL database you can ask a page of the whole result from the DB: the kea-server will translate this page to JSON and sends to the requestor.
with Cassandra you do not have this so you can get the whole result and page it in the kea-sever, etc.

To summarise, what matters is where the paging is done. In #313 (closed) we decided to do it only in the DB (so only for SQL DBs).

This ticket is to explore what we could to meet the use-case need (similarly or differently) with the Cassandra back-end

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