[2.2] Random byte[] for start entry header to randomise tx checksum

This PR adds a random `byte[]` to transaction start entry's additional header field to randomise checksum. This is needed because current checksum is calculated only from txMasterId, txAuthorId.
@jakewins I don't think the it's worth it. ThreadLocalRandom is pretty close to the same performance even though it adds a ThreadLocal-lookup on top, and it gets even faster on Java 8: http://chrisvest.name/examening-the-performance-of-xorshift.html