GRAILS-10086 Replace usage of SimpleTypeConverter with ConversionService

http://jira.grails.org/browse/GRAILS-10086

Please review the changes and merge this pull request if it's ok.

grails-hibernate-plugin requires 2 commits from the GRAILS-10086 branch:
https://github.com/grails-plugins/grails-hibernate-plugin/commits/GRAILS-10086