Implement parallel building

[wolfv]

With the ROS rebuilds we could still save ~50% of the time if we would execute builds in parallel.

Lot's of time is taken by linking packages into an environment, and by running low-cpu stuff like cmake scripts.

We can create a graph of all recipes, and run all in parallel whenever possible.

[wolfv]

There are some additional interesting possibilities with the Jobserver plugin to GNU Make (used by catkin_tools). We can implement things similar to catkin_tools to run make in parallel over multiple packages.