Render timeout too short for RasPi2 platform

[sonntam]

See here.

The currently hard-coded render timeout of 10 seconds is too short. Maybe change the rendering so that no hard-coded timeout is necessary by observing the spawned process.

[sonntam]

The default node's state machine can take as long as 40 seconds to render on a Pi2 according to this post.

[sonntam]

As stated in sverweij/state-machine-cat#116 using native graphviz would be a good way around the performance issue since rendering is done in the runtime - not in the client browser.

[sonntam]

A couple of new features have been added:

• Caching functionality that only need to redraw graphs if something changed or if they became too old (TTL of 4 weeks)
• Ability to use graphviz's native dot renderer program. In order to use it it has to be available on the path.