Thread safe graph interface

[ErikNatanael]

Graphs should be controllable through a thread safe Sync interface. This can happen via a message passing interface to a control thread.

• Schedule any change to a graph via messages
• Pass back keys and requested data to user thread(s)
• Handle fallible operations (custom error handler?)
• Maintain scheduling accuracy of relative time
• Efficiently commit changes and update scheduling automatically
• Schedule changes to Resources used in a running Graph

This interface should work also in a single threaded context.

[ErikNatanael]

Closed by #12