MB-9844 Thread-safe calculation for estimatedcount

Change-Id: I21243cb62eb6efc1651e601c2354f3d30de3b185
Reviewed-on: http://review.couchbase.org/31674
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>