Clustering : Commits from the journal should be batched to improve recovery time

To reproduce the problem, do the following,

1. Run a karaf distribution of the controller. OpenFlowPlugin should be just fine.
2. Install the feature odl-mdsal-clustering
3. Add 50,000 flows and remove 50,000 flows using the attached script.
/usr/local/bin/python flow_config_perf.py --nflows 500 --nthreads=10 --ncycles 10
4. Stop the controller
5. Start the controller
6. Once the controller "appears" started try accessing the inventory - note that it fails.
7. After about 5 minutes the inventory becomes accessible again.