Split AsyncIoProvider into high-level and low-level interfaces, so that alternate EventPort implementations can implement the low-level interface and reuse the higher-level stuff.

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.

Eliminate the ability to have multiple threads working on building the same message -- performance penalty is too large, and applies even to single-threaded users.

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 opt-in cancellation for local calls.  (Previously, cancellation would happen whether or not the callee had opted in.)

Don't rely on default assignment operators for cap clients since they break with double virtual inheritance.