CBQE-1096: cache active task stat checker

This class should be cached to keep track
of index behavior between checks

sample test included

Change-Id: Ie71c43169b092420120c58d3c2b3fbf129f6fc5d
Reviewed-on: http://review.couchbase.org/25384
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>