implement streaming requests
require that write() cannot happen after done(), and validate the number of bytes if known
upgrade to node-capnp 0.1.5, improve error handling for streaming requests
Introduce Cap'n Proto interfaces for HTTP APIs.
Merge branch 'streaming' of github.com:dwrensha/sandstorm into inbound-streaming
Clean up inbound streaming. Some issues still present (see TODOs).
Further improve streaming now that we can detect when a capability is remotely closed.