Clarified the expiry time format (Please validate the assumption before merging).

1. Mentioned that the binary format for expiry is little endian.
2. The length encoding is 1byte, 2byte or max 4 byte (In which case we will ignore first byte). While I guess in case of expiry timestamp it is either 4 byte or 8 byte, both cases, the full length will represent the value. So, my assumption is that the expiry timestamp doesn't really represent the length encoding format (Please validate before merging). So, removed the reference to length encoding from expiry.