core : Persist engine session on login and delete on logout

Persist engine session into database on user login
and clear the engine session on user logout

Change-Id: Id716d9c6f65898ccd0ffd84d8b05926b78ac9692
Bug-Url: https://bugzilla.redhat.com/1092744
Signed-off-by: Ravi Nori <rnori@redhat.com>