Eliminate the concept of imbuing messages in favor of the simpler concept of setting a cap table directly on MessageReader / getting one from MessageBuilder.  This eliminates capability-context entirely.  This was made possible by the earlier change which moved capabilities to a separate table rather than storing CapDescriptors inline, but I didn't realize it at the time.

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.

In RPC protocol, rename request -> params, answer -> results.  Also fix up Join stuff in rpc-twoparty.capnp, because it was sort of wrong.

Implement two-party network.  The first RPC call over a socket took place at 2013-11-08 14:46:43 -0800 and completed successfully.