refactor(cassandra): Mark queries as idempotent to enable retries

[abhinavk96]

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• The commit(s) message(s) follows the contribution guidelines ?
• Tests for the changes have been added (for bug fixes / features) ?
• Docs have been added / updated (for bug fixes / features) ?

Current behavior :
Idempotence is not explicitly set on any prepared statements for the queries.

Datastax specifies that by default queries are not marked as idempotent unless overridden at the session level.
Because of this the queries won't be retried according to the retry policy.

Reference: https://docs.datastax.com/en/developer/java-driver/3.6/manual/idempotence/#query-idempotence
Quote from above:

Idempotence matters for retries and speculative query executions. The driver will bypass those features if the Statement#isIdempotent() flag is set to false, to ensure that the statement does not get executed more than once.
In most cases, you must set that flag manually.

New behavior :
Setting idempotent explicitly to true will enable the retry policy to act on these queries.

Note:
The queries have all been deemed to be idempotent. Please review individual queries carefully to verify that assertion.

[vishramachandran]

LGTM