Add a supported way to use WritePartitioner from Execution

[johnynek]

We can take a series of writes in the Typed API, or we can use a FlowDef to create an Execution which has been broken into small pieces so cascading can plan it, even if it is very large.

We don't currently have a way to execute an Execution[A] such that all writes are partitioned before we go forward. This may be less accute in the Execution API since users can usually find choke points to put forceToDiskExecution, but still it would be nice to have a config flag or something that could flip it on for the entire Execution.