handler: run event loop within executor

[jkarneges]

Now that the necessary APIs have been made available to the C++ code, we can run an EventLoop that executes asynchronously within a task spawned on an Executor. We'll start out by doing this in the handler.

Moving the existing straight-line logic into the setup and done closures is a little tricky regarding lifetimes. The closures borrow, so we need to make sure the loop, deferCall, and engine bindings all live outside them, and we need to ensure correct construction/destruction ordering. Notably, DeferCall must not be constructed until after an EventLoop exists (i.e. after setup called), and it needs to be destructed before the EventLoop goes away (i.e. before done ends).