Synchronous writes and calls to fdatasync(), fsync() or
FlushFileBuffers() would ruin performance. So, let us
submit asynchronous writes for the doublewrite buffer.
We submit a single request for the likely case that the
two doublewrite buffers are contiquous in the system tablespace.

buf_dblwr_t::flush_buffered_writes_completed(): The completion callback
of buf_dblwr_t::flush_buffered_writes().

os_aio_wait_until_no_pending_writes(): Also wait for doublewrite batches.

buf_dblwr_t::element::space: Remove. We can simply use
element::request.node->space instead.

Reviewed by: Vladislav Vaintroub