In order to be able to allow jobboard object users
to know when a job was last modified or when the
job was created we need to provide accessors to make
this possible. These can be used to claim a job after
a given period, or for general tracking of which are
the oldest jobs...
Implements: blueprint job-reference-impl
Change-Id: I467bb083d0b143826a44c6aeb6499c483b88fe65