MB-8370 add WorkLoadPolicy class

This new class defines and applies user defined workload
access pattern while global io manager launching worker
threads for a newly registered bucket.

Change-Id: I1905786fc67b9004118f11d83196d76f95dfb136
Reviewed-on: http://review.couchbase.org/26723
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Jin Lim <jin@couchbase.com>