Reduce threadCount and iterations in secondary_reads.js workload

The snapshot_reads.js workload presently uses a threadCount of 50 and number of iterations of 40. It can presently run from 40 minutes to up to 1 hour on some distros. The test was not specifically designed to necessitate a high count of threads and iterations, so these should be reduced to lower the execution time of this test. Additionally, an index can be added on "x" to facilitate the sort queries on x.