Refactor ExecutorPool out of scheduler.cc/h

Change-Id: Ie67a093ebcd1ed3a645847810f2050439e9283ed
Reviewed-on: http://review.couchbase.org/31721
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>