unaligned requests are not handled async by AIO Sendfile
When a request is made for a file using the network_linux_aio backend an mmap() is setup, however when the request is unaligned (for example seeking in a video with mod_flv_streaming) the backend drops back to sync sendfile.
This causes a huge performance hit for video streaming sites. This patch forces the request to be aligned by padding the read offset to the previous page boundary
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