[INTERNAL] removal of a user is now synchronized by the host

Currently a leave message was send to all users. The patch ignores
all leave message that are not sent from the host side. Furthermore
the host will now synchronize the user list if a user is removed from
the session.

Change-Id: I8cf09b354d17b0e24907b7697b0253aa5a0e6f38
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1028
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>