CBQE-819: min ops throttling

some times op throttling is too agressive,
minimum a worker should be able to handle is 5k
unless cluster/worker isn't stable

Change-Id: Ie963b971564f2dbd096c4a230b424bde9acd1019
Reviewed-on: http://review.couchbase.org/24573
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>