This is necessary when someone other than the promise owner might need to destroy the state a promise is operating on. It comes up when implementing pumpTo() for userland pipes -- cancelling the pump needs to cancel any writes currently passing through the pipe.

Fixes #286.

I intended to do this, like, 4 years ago.

These are useful in unit tests. Often, some tasks have been queued, and we want to test that a particular thing happens or doesn't happen as a result of those tasks, without actually waiting on any specific promise. We need a way to run the event queue until it is empty. For that, we add WaitScope::poll().

Or, sometimes, we want to check if a specific promise is (or is not!) resolved as a result of the tasks that have been queued so far, but we may not want to actulaly wait() on it for a couple reasons:
- We may actually want to verify that the promise is *not* resolved by tasks so far. wait()ing on it would be expected to deadlock.
- We may expect the promise to resolve, but would prefer not to deadlock if it doesn't resolve (we'd rather fail fast).

For this, we add Promise::poll().

There are lots of tests I've written which could be simplified by this, but for now I'm not refactoring any existing tests.

Note that adding these (particularly Promise::poll()) required updating the async framework a bit, in that this is the first case where PromiseNode::onReady() might be called multiple times, or might be canceled without destroying the PromiseNode entirely. Luckily this was not hard to account for. (We still have the rule that get() can only be called once, though.)

- Reduce noise.
- Trace through async callbacks.

This is not currently used, but eventually we'll use this to support efficient inter-thread messaging.

That said, I've decided that inter-thread messaging is complicated and I don't want to work on it right now.

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Optimize promise tail calls by making ChainPromiseNode automatically detect and remove redundant nodes.

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.

Implement exclusive promise joining:  Join two Promise<T>s to create a Promise that resolves when either input resolves.