Change Win32 O_SYNC method to O_DSYNC because that is what the method

Change Win32 O_SYNC method to O_DSYNC because that is what the method
currently does.  This is now the default Win32 wal sync method because
we perfer o_datasync to fsync.

Also, change Win32 fsync to a new wal sync method called
fsync_writethrough because that is the behavior of _commit, which is
what is used for fsync on Win32.

Backpatch to 8.0.X.