Cleaner disconnect handling.  Better fix for issue #71, and also simplifies the interface and improves robustness.

Make TwoPartyVatNetwork::onDrained() work correctly on the server side.  Previously only worked on the client side, where it's actually not very useful.  Fixes #71.  There's a deeper problem, though, that simply holding a capability received from the client will hold the dead connection open.

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.