Remove the use of MD5 in org.apache.avro.file.DataFileWriter#generateSync

In the chat, someone mentioned using a FIPS environment, which disallows the use of insecure cryptographic hash functions, like MD5.

The DataFileWriter class uses an MD5 hash of a random UUID and a timestamp to generate what's essentially 16 random bytes.

This can more easily be done with SecureRandom.