[threadpool] KUDU-3364 Add TimerThread for thread pool

Add TimerThread to ThreadPool to support
executing tasks asynchronously and with a delay

I plan using the feature to add tools to trigger data rebalance,
leader rebalance, some kinds of compaction ops. Of course, some
single threads can refactor into one threadpool.

More details at KUDU-3364.

Change-Id: If51fce48ae6a45a0bf3314f8a102ee373c5c442e
Reviewed-on: http://gerrit.cloudera.org:8080/18447
Tested-by: Kudu Jenkins
Reviewed-by: Yingchun Lai <acelyc1112009@gmail.com>
3 files changed