MB-12067: PathoGen 'Frozen-mode'

Add a second mode to PathoGen - randomly stop changing a small
percentage of documents after each size increase, resulting in an
(increasing) number of documeents being 'frozen' at their last size
(and crucially at thier last address inside memcached).

This exposes the overhead (compared with mem_used) resulting from
using size-class based allocation.

As part of this stagger which worker input queue documents are
initially dispatched from. This ensures that a particular worker (and
crucially it's memcached front-end thread) performs set()s of all the
different sizes.

Change-Id: I32439b0c84b6512c6fdb6b5a5241e7d01566415e
Reviewed-on: http://review.couchbase.org/40928
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Venu Uppalapati <venumahesh@gmail.com>
Reviewed-by: thomas anderson <thomas.couchbase@gmail.com>
Tested-by: thomas anderson <thomas.couchbase@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>