JavaBeans persistence style when possible

This branch tries to change some parts of the persistent classes which inherit from other classes. The motivation is basically that in our case this approach makes a much clearer code in some cases (see PersistentMessage).
Summary of changes:

* common-utils/.../Message.java:
  * It's not longer an interface but the default implementation.
  * DefaultMessage disappears

* PersistentMessage
 * Inherits from Message and now uses getter/setter based persistence


* PersistentJob 
 * getter/setter based persistence when possible (everything but id)


* PersistentJobContext
 * getter/setter based persistence when possible ( not in the case of complex persistent classes such as persistent ports, mapper, etc...)
 * Complex hydration/dehydration has been moved to a external class (ContextHydrator)