Implemented _sessionid

Will resolve https://github.com/jbeard4/SCION/pull/289#issuecomment-57415737
This API should be passed in from the environment. This will be provided by this library: https://github.com/jbeard4/SCION-COMM

Issue created for this here: https://github.com/jbeard4/SCION-CORE/issues/7