[FIX] for commit 45edbd76cec29b67ceb9b5e88bcb82b6217adc15

The outgoing sequence number was not reseted if a user
left the session. 

Change-Id: I5b05fbd1e61bb87bbe47274894b284ff36fb6948
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/873
Tested-by: Jenkins CI
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>