Allow RevisionTimestamp to be of type java.time.LocalDateTime

Currently the RevisionTimestamp must be of type long, Long, java.util.Date or java.sql.Date. But - following Hibernate's general support for Java 8 date and time types introduced in Hibernate 5 (see HHH-8844) - it would be great to have RevisionTimestamp support java.time.LocalDateTime as possible type as well.