fsync instead of calling `Write`

[Crypt-iQ]

Write doesn't sync to disk so a crash can corrupt data. One example of this is here:

neutrino/headerfs/file.go

Lines 19 to 25 in 4bc2c99

	func (h *headerStore) appendRaw(header []byte) error {
	if _, err := h.file.Write(header); err != nil {
	return err
	}
	return nil
	}

I forget if there are other call-sites

[Chinwendu20]

Nice. We would just add, file.Sync() for the appendRaw method in headerStore, CFheader store, filterdb store.

[mohamedawnallah]

Hey @Crypt-iQ I've submitted a PR for this issue. I'd love to receive any feedback! 🙏