Fsync frame data in wallog before writing header

Currently in logger.rs we are not issuing any fsyncs at all.
We do `flush()` in `write_header()` but this is a no-op on Unix
because there are no userspace buffers to flush since `write_all_at()`
writes directly to kernel.

Not fsyncing can cause the following bug:

- We write X frames
- We write wallog header (containing frame count X)
- Kernel reorders writes so that header has been written out
  but not all of the frame data
- Crash
- Sqld starts up, reads wallog header which claims X frames
  exist in wallog, but only Y frames (Y < X) have been written
- short read in read_frame_byte_offset_mut() returns an error
  and crashes the server

It's also important to sync after writing frames but before
writing the header so that there is no write reordering scenario
where the header is persistent before the frame data is.