At submit_buffer, fixed writer selection makes throughput bottleneck.
Fix to select a writer by rough round-robin,
 * to reduce bottlenecks
 * not to reduce performance without locks

What is important is to select a different writer from previous submits, not to select "strictly next" writer.
That is why this patch doesn't include locks.
Merge pull request #245 from tagomoris/flush_threads_roundrobin

Select writer threads by round-robin