Flush before-images first

The before-images of modified pages are flushed to disk before the modified pages themselves.

In practice, the physical-log buffer is flushed first and then the buffers that contain modified pages. Therefore, even when a shared-memory buffer page must be flushed because a user thread is trying to acquire a buffer but none is available (a foreground write), the buffer pages cannot be flushed until the before-image of the page has been written to disk.

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