additional fix fro BUG-782 unregistering switch providers

- CompositeObjectRegistration is stored into sessionContext upon addSession
- upon removeSession the CompositeObjectRegistration is closed

Change-Id: Ieee90f50645db12c5df198fd4501daa804a135d3
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Merge "additional fix fro BUG-782 unregistering switch providers"