The existing ThreadPoolExecutor was misconfigured and caused
the thread pool to recycle idle threads continuously. VisualVM
showed that a thread got recycled about every 3 seconds, creating
and destroying several hundred threads in about 15 minutes.
Use ExecutorService.newFixedThreadPool instead, as recommended
by the javadoc for ExecutorService.
Add a new method to the worker requestShutdown that allows the worker to
gracefully shutdown all record processors. The graceful shutdown gives
the record processors a last chance to checkpoint before they're
terminated.
To use these new features the record processor must implement
IShutdownnotificationaware.
Allow disabling the shard sync at startup if the lease table already
contains leases. This will reduce the startup load for larger streams
when restarted the Kinesis application.
