output_buffers value breaks thread_pool/aio on 1.9.5+

[Richard Kearsley]

Hi
Since 1.9.5,
*) Change: now the "output_buffers" directive uses two buffers by default.

The two buffers do not work with thread_pool/aio, the connection is 
closed at 32,768 bytes (size of one buffer)

These messages are shown in the error log:
[alert] 126931#126931: task #106 already active

- Changing output_buffers back to 1 32k avoids the problem
- aio=off avoids the problem

Many thanks
Richard