Coprocessor task monitor

The coprocessor tasks are managed by the TaskMonitor in the server side. Usually these tasks run in a random long time, it’s hard to know how many tasks and how about the task status in the HRegionServer in the client side.
To add the client API for the coprocessor task monitor could improve that. Users could find the coprocessor task status by categories(coprocessor, client, user name, etc.), this is helpful to improve the lives of the operations guys who have to run HBase in production by giving them an API that can integrate HBase server and coprocessor task monitoring with Nagios etc.