Testing optimization of leaf with _common paths.

We would need a different bertha execution strategy for this
optimization to work, I think. When a _common: paths[] is provided,
bertha will serially execute the common path traversal before moving on
to further resource requests. However, because we are creating a new
execution plan for each resource, this _common is duplicated and thus
the optimization step is lost.

We need a better way to share execution plan results with siblings so
they may benefit from a fetch. @mnichols let me know what you think, or
if this makes sense.