Function write_cold_record 

pub fn write_cold_record(
    out: &mut [u8],
    kind: Kind,
    enc: Encoding,
    at: Addr,
    len: u32,
    expire_at: Option<u64>,
)

Write a record saying the value is at at on the file and is len bytes.

out must be exactly cold_record_len long. kind and enc are carried across from the record being replaced so that the questions which can be answered without the device still can be.