PHOENIX-7528 Add metrics for overall wall clock query wait time in Phoenix client thread pool and total time spent in executing HBase scan tasks

[sanjeet006py]

Summary of the change:

• Introducing two new metrics for tracking wall clock wait time of a query in Phoenix client thread pool and wall clock time spent in executing HBase scan tasks in Phoenix client thread pool.
• Also, made the change to use JobCallable in RoundRobinResultItr. when submitting next batches of scans as scanner cache of previous batches of scan has been exhausted. By using JobCallable we can track total wall clock time spent by a query waiting in Phoenix client thread pool queue along with total wall clock time spent in executing tasks (end to end).