API Plan optimizations is needed

[acmol]

We should add a optimization layer in the current API layer, it could be called API Plan layer.

At this moment, API layer will transform the user's code to a LogicalPlan directly, and some information is lost, such as LogicalPlan don't know what is join, the LogicalPlan only know that two nodes are cogrouped and then a Processor will process the two cogrouped result.

Eg.

pc1.distinct().join(pc2)

is equal to

pc1.cogroup(pc2) \
       .apply_values(lambda p1, p2: p1.distinct().cartesian(p2)) \
       .flatten_values()

But we can't optimize it automatically without the help of API Plan.

So, API Plan is meant to keep all the information we can get from user's code, and optimize the plan by the information.