bitbake: toaster: do not save objects in session

bitbake: toaster: do not save objects in session

In order to avoid problems when using JSON serializer for
saving sessions, we move from storing the objects themselves
in the session to storing the object id and reloading the
object when retrieved.

This allows, for example, to use cookie-storage sessions if the
infrastructure owner so desires.

(Bitbake rev: 39d0f0c2e87d4b161f1eeaa2657e61b5a6bc9ee2)

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>