Make all Promise methods consistently consume the promise (returning a new promise when it makes sense), rename daemonize -> detach, and make eagerlyEvaluate() require an error handler (this caught several places where I forgot to use one).

Overhaul the way EventLoop is specialized so that it's possible to hook up to an existing event loop infrastructure that is not KJ-aware.  This also makes the async IO API more dependency-injection-friendly.

Revamp concurrency model, part 1:  EventLoop no longer allows cross-thread event queuing, simplifying many things.  Capability clients are no longer thread-safe, so they don't have to be so const.  In the future, explicit ways to communicate between threads will be re-added, but threads will be treated more like separate vats that just happen to have a particularly fat pipe.  Upcoming:  Remove mutexes.

Make EventLoop current when constructed, so that you can use the Promise methods even when the loop hasn't actually started yet.

Add tests for actually receiving multiple signals and multiple polls at once, not just listening for them.