MB-10372: Make estimateTime in warmup class atomic

Change-Id: Ie362b00f9d56bb94a62b562610dd9be2dfee68d5
Reviewed-on: http://review.couchbase.org/34376
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>