pillowfight: allow sequential access pattern

This patch allows the pillowfight utility to be used as a simple loader
which generates and loads documents in sequence. As part of these
changes:

* Individual worker threads operate on individual items using a 'divide
  and conquer' strategy.

* The 'populate' phase is no longer a separate piece of code. It
  operates "as if" the workload was running with the equivalent of the
  workload performing only sets.

Change-Id: I8b0c51a792f4b5f062242fdc58ed8d085c974fa8
Reviewed-on: http://review.couchbase.org/41948
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>