Description
string conversion to time might be costly operation which could be moved to private final class member field. the same goes for the timeFormat which is unlikely to differ across the calls to the Ctime udf.

Notes:

• timeformat is final and set in AbstractConvertStep and can be moved to the class
• Conversion could possibly be done using functions.from_unixtime from spark without the need to use custom function
• time format is non-standard we should consider switching to use ISO8061