Improve logging with warmup's parallelization

Change-Id: Ia450f2d9f134560968cb0e624a35b33376939c81
Reviewed-on: http://review.couchbase.org/34950
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>