CASSANDRA-19177 Ensure resources are released after index query timeouts

[mike-tr-adamson]

patch by Mike Adamson; reviewed by for CASSANDRA-19177

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The Cassandra Jira

[mike-tr-adamson]

This patch makes the checkpointing of the index query more granular but ensures that any resources being used are correctly released if there is a timeout.

I have also deleted two classes that were not being used.