CBQE-1152: query workload throttling

this change checks for buildup in queue with pending http requests (ie...query_multi_default)
and also queues corresponding with any query tasks.

Change-Id: I9a99295bc8ee4adbc2143c4a247c784eb974fe55
Reviewed-on: http://review.couchbase.org/26936
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>