[INTERNAL] added new ConnectionStateListener interface

Some logic in Saros does not care what connection is used but is only
interested in the state transition. Added a new interface so that other
connections can be established.

Change-Id: I9522aa6ac438b026d5b92d9d8acaa02452a865bc
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1591
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>