CBQE:0 Add support for bucket priority in xdcr tests

Low priority and high priority buckets enqueue their tasks in different queues( of
             different  priorities). With Global IO manager, tasks from all buckets are serviced by the same number of threads. However, tasks in low priority
             queues are polled less frequently by the global threads than the tasks in high priority queues. This behavior differs from 2.x in which each bucket
             had a dedicated number of worker threads(provided by user specified configuration).  The values were between 2 and 8. So the mapping between
             thread numbers and bucket priority is mainly to preserve backward compatibility with how ep-engine managed the relevant configuration settings in
            2.x. In 3.x, any bucket with a “number of worker threads” setting <5 is assigned Low priority and >=5 is assigned High priority. But, the total number
            of worker threads spawned in the Global pool is not impacted by these settings. The total number of threads strictly depends on hardware specs.

Change-Id: I808aa62212c5c42751d4c0bcde3e63f857bb5199
Reviewed-on: http://review.couchbase.org/39193
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>