Enable Direct I/O

Hi,

JDK 10 introduced a new API for Direct IO that enables applications to bypass the file system cache and potentially improve performance. Details of this feature can be found at https://bugs.openjdk.java.net/browse/JDK-8164900.

This patch uses the JDK 10 API to enable Direct IO for the Cassandra read path. By default, we have disabled this feature; but it can be enabled using a  new configuration parameter, enable_direct_io_for_read_path. We have conducted a Cassandra read-only stress test and measured a throughput gain of up to 60% on flash drives.

The patch requires JDK 10 Cassandra Support - https://issues.apache.org/jira/browse/CASSANDRA-9608 

Please review the patch and let us know your feedback.

Thanks,

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