use java.time.* data/time api classes for dates and times in generated java

[pahjbo]

To help to minimise any timezone confusions it would be good to alter the generated code to use the java 8 date time classes to interact with the database.

[pahjbo]

there is an initial implementation using LocalDateTime on the dateTimeImplementation branch - not sure that it makes things better though as the dates should be serialised/store as UTC, and perhaps java.util.Date with no timezone handling is actually less confusing.....