Extract TaskQueue out of scheduler.h

Change-Id: If46478214e81defa42d17f3e7c63f639ac266756
Reviewed-on: http://review.couchbase.org/31720
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>