Li - ofp dangling connections after mininet switches disconnects

How to reproduce :

• start mininet and connect 15 switches (tolology tree,4)
• using flow_config_blaster.py (/integration/test/tools/odl-mdsal-clustering-tests/clustering-performance-test) blast 40 thousands flows (10 threads 4000 flows/ switch)
• wait after statistics collects data
• optional step : issue seems to happen everytime after garbage collecting occurs, let setup run for approx 10 minutes, there can be disconnecting/reconnecting seen in DS
• disconnect mininet
• read oper DS