Write object headers with stored file extents

Previously file object headers were being written with file extents
rather than stored extents.

This could lead to situations where the data in the cache was not yet
written to flash but had been accounted for in the object header file
size.

Under power fail this could cause zero-filled blobs at the end of a file.

This change tracks the stored file size and only write object headers
reflecting the extents of the file that has actually been stored to flash,
thus avoiding the holes.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>