Confirm that we can (rolling) upgrade from 2.0.x and 2.1.x to 2.2.x after HBASE-20881

hbase-2.2.x uses a new Procedure form assiging/unassigning/moving Regions; it does not process hbase-2.1.x and earlier Unassign/Assign Procedure types. Upgrade requires that we first drain the Master Procedure Store of old style Procedures before starting the new 2.2.x Master. The patch here facilitates the draining process. On your running hbase-2.1.1+ (or 2.0.3+ cluster), when upgrading: 1. Shutdown both active and standby masters (Your cluster will continue to server reads and writes without interruption). 2. Set the property hbase.procedure.upgrade-to-2-2 to true in hbase-site.xml for the Master, and start only one Master, still using the 2.1.1+ (or 2.0.3+) binaries. 3. Wait until the Master quits. Confirm that there is a 'READY TO ROLLING UPGRADE' message in the master log as the cause of the shutdown. The Procedure Store is now empty. 4. Start new Masters with the new 2.2.0+ code.