[INTERNAL] replaced JID with User objects

Replaced the JID objects in the concurrent package and the
ActivityQueuer with User objects as the logic should not care about how
a user is connected to the current session.

Change-Id: I1085815aeecf1c4ddf45f396eb838faade2935ac
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1608
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>