Reconsider process creation primitives on Windows

Windows does not have the same notions of process hierarchies as Unix, and so killing groups of processes requires us to make sure all processes are contained in a job object, which acts something like a cgroup. This is particularly important when we decide to kill a task, as there is no way to reliably do this unless all the processes you'd like to kill are in the job object.

This causes us a number of issues; it is a big reason we needed to fork the command executor, and it is the reason tasks are currently unkillable in the default executor.

As we clean this issue up, we need to think carefully about the process governance semantics of Mesos, and how we can map them to a reliable, simple Windows implementation.