CBD-1148: Migrate all NonIO tasks into Common Pool

This removes per-bucket threads from the NonIO dispatcher
into the global thread pool shared by all buckets
Following tasks handled by the non-io thread are now migrated
into the common shared thread pool...
1) Backfill task
2) Hash Table Resizer
3) Checkpoint remover
4) ExpiredItemPager
5) Item Pager
6) VBCBAdaptor (async task spawned by daemon tasks 1-4)
7) Tap Connection Notification (daemon task)
8) Tap Resume
9) Tap Connection Reaper
10) VBucket Memory Deletion
11) FlushAll task
12) Checkpoint Stats
Also removed isDaemon and makes GlobalTask's starttime internal
Allow new api in ExecutorPool to stop a task group entirely
Change-Id: Ic5cbbfb6d1a140335b0ff1a7c7dc31e20ca47186
Reviewed-on: http://review.couchbase.org/30775
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>