Cbench throughput mode kills the MD-SAL data store

To reproduce:

1. Put the drop test feedback to go through the MD-SAL Data Store. From the controller OSGI console, type
dropAllPackets on

2. Run cbench in the throughput mode, for example:

cbench -c 192.168.162.1 -p 6633 -m 10000 -l 10 -s 16 -M 10000 -t

The throughput is very low (about 2.6% of the latency cbench throughput going through the data store). After the test finishes, the CPU utilization remains very high for several minutes (I had to kill the controller after 5 min, because I ran out of time). This indicates some massive queue buildup in the system.

I am adding thsi bug to MD-SAL because the throughput mode through RPC loopback is table, and performs very well. It's the loopback through the data store that knocks out the controller.