Enhancement Request: Use java.time instead of java.util time classes

Starting with Java 8, better time classes are available, that could also be used for methods like _message.getReceivedDate()_

To preserve backwards compability, a new method could be added (something like _get...LocalDate()_). This would allow new programs to use the java.time classes consistently.

Start with the Message.getReceivedDate() entry point mentioned in the issue and review the existing time-returning API. Define a backwards-compatible java.time-based method and document the intended naming and compatibility behavior; done means the API clearly supports java.time without removing the existing method.

Written by the indexing model from the issue text.