MB-8244: Add dispatcher, worker thread stats

- Enables dispatcher stats
- Prints reader-writer thread stats
- Add condition to return immediately without
printing stats if engine in shutdown mode
- Changes based on previous comments included
- Adding a unittest: test_worker_stats
- Formated dispatcher stats (sample)

 iomanager_worker_0
    runtime: 6ms
    state:   running
    task:    Running a flusher loop

- Add stats' details in doc/stats.org

Change-Id: I034dc6ea04df0e7e018849e1ebe8c1f6a02d9246
Reviewed-on: http://review.couchbase.org/27121
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>