Refactor TaskLogEntry out of scheduler.h

Change-Id: Ie7b1c95c8da3a0ea62c738c4eb2288c3e1664cdf
Reviewed-on: http://review.couchbase.org/31724
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>