Change dateCreated and lastUpdated fields to java.time.Instant

[skiwi2]

The dateCreated and lastUpdated fields in all domain classes should be changed from java.util.Date to java.time.Instant.

This can happen from Grails 3.1 onwards, which has Hibernate 5 support that supports java.time.Instant, and it is scheduled for Q1 2016.

Please make sure that existing dateCreated and lastUpdated fields get migrated to the new java.time.Instant correctly without loss of data.