Statistics collection slow - performance

Statistics collection performance regressed from Helium SR1. In Helium SR!, statistics for 5k flows are collected in ~4 seconds. On master built today, statistics collection for 5k flow times out after the 100 seconds. It think it takes about 15-20 minutes to do all the round-robin collection.

To reproduce: run the integration md-sal performance test flow_add_delete.py:

./flow_add_delete_test.py --flows=1000 --threads=5 --bulk-delete --auth

and observe the line 'Stats collected in <xyz> seconds.' xyz for Helium SR1 and for the master should be the same if run on the same hardware.