[INTERNAL] moved ISarosSessionManager interface to Saros/Core

Had to temp. disable one method. Must be re-added laster.

Change-Id: I806d397251a2aa1f1dcee2b8847cb4c43e4923ef
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1950
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>