make logout service use the current pseudonym

if a user has multiple pseudonyms the logout service
may not work because the callbacks may be registered
to a different pseudonym.

Test Plan:
 * make multiple logins for a user
 * use the logout service
 * the callback should always be pinged

closes PLAT-747

Change-Id: I54f599f96fad4ecfa264452360e6862191f9d826
Reviewed-on: https://gerrit.instructure.com/43841
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>