CBQE-819: ops throttling when workers overcommited

every 10 seconds run a task that manages ops per second of a workload
(See workload_manager.kv_ops_mgr)
Purges extraneous tasks and lowers current ops if workers are slow in processing new tasks.

Change-Id: I2549fbecbd21fa64fd62e244528060cb103b4a93
Reviewed-on: http://review.couchbase.org/24517
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>