Enable hibernate second level cache for org.openmrs.Privilege

With the normal Bahmni workload, we have found that 'mysql privilege select' takes larger share in the total number of database hits.

We ran a same set of gatling tests before and after adding ehcache for privilege and found that the average response time has reduced and throughput has increased thrice. Below are the images:
Before cache:

After cache:

We propose adding Privilege to the Hibernate second level cache to see the above performance improvement. Also we would like this to be ported back to 2.0.x