Fix SessionManager bug where invoking callbacks in loop could cause crash.
Fix SessionManager bug where invoking callbacks in loop could cause crash.

Make SessionManager copy set of disconnected objects before starting
to invoke their callbacks. Previously it was looping over a set of
objects and invoking each callback, but the callbacks could (and did)
make requests back to the SessionManager which affected the data being
looped over. Making a copy ensures all the callbacks are invoked.
Fixes #255 since the notification was already being made, but was
being missed because the client ended up crashing before it could be
seen.

sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata,sirikata/sirikata