Use readable and sortable file names for UDF logs

[kaklakariada]

When UDF logging is activated with system property test.udf-logs, the project generates log files named target/udf-logs/udf-log-294787982703928309.txt. The number in the file name is random and it is not possible to sort the log files by file name to get a chronological order and to find the relevant file.

To fix this we should

• use readable file names that include an ISO timestamp
• use file names that are in chronological order when sorted alphabetically