Multithreading

[maurer]

In my original design (back when Holmes was still a server) I had multiple processing threads. I think this would be valuable to bring back because:

• It may work well as an abstraction (e.g. for Make run_rule iterative instead of recursive #15 by having different threads manage different rules)
• It gives an obvious opportunity for a performance gain by running multiple external functions simultaneously
• It can be used to provide partial fault tolerance so that if an external function crashes, it doesn't need to bring down the whole Holmes process, just that rule.
• It will pave the way for adding server functionality back at the end of the project.

[maurer]

#33 removed all the reasons this was a release blocker - it solved #15 with async tasks, and gave the library code the structure it will need to be used as a server. Eventually, we'll want to add true multithreading (partially implemented in c899a28 with lifetime errors), but I'm removing it from the release milestone since the release blocking parts are gone.