[STF] changed connect behavior

It is now possible to control how a connection should be established.
This patch allows to ignore connection attempts if a connection
is already established with a given account.

Change-Id: I2b31a82b51a9c82f61e66e8ec1fe34ae94e99bee
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1501
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>