Merging r4195, r4196:

Merging r4195, r4196:

AIO related fixes:

*) Fix for connection drops with AIO.

   Connections serving content with AIO to fast clients were dropped with
   "client timed out" messages after send_timeout from response start.

*) Fix for socket leak with "aio sendfile" and "limit_rate".

   Second aio post happened when timer set by limit_rate expired while we have
   aio request in flight, resulting in "second aio post" alert and socket leak.

   The patch adds actual protection from aio calls with r->aio already set to
   aio sendfile code in ngx_http_copy_filter().  This should fix other cases
   as well, e.g. when sending buffered to disk upstream replies while still
   talking to upstream.

   The ngx_http_writer() is also fixed to handle the above case (though it's
   mostly optimization now).