test: Add option to flush memtables for perf_simple_query
test: Add option to flush memtables for perf_simple_query

The test in question measures the speed of memtables, not
the row_cache. With this option it can do both.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20200507140603.12350-1-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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