Refactor Zeus' core process management

[metcalf]

This replaces the core of Zeus' process management (the processtree package) with goals of:

• Substantially narrowing the interface of the package to make it easier to reason about.
• Breaking process management down into separately testable units.
• Resolving some of the race conditions (and awkward mitigations for races) in the current code.

I believe I realized a few other benefits in the process:

• Improving cleanup so that Zeus terminates its processes before exiting.
• Better propagating potentially fatal errors in process management.
• Removed global state from the status chart package.

TODO:

• Some manual QA
• Release an internal candidate for deeper beta testing
• Consider whether there's a way to make the node and process packages internal to processtree.
• Update design docs.

[metcalf]

Closing for now, given the major issues with Zeus' current file monitoring system I'm going to have to rethink a bit.