Streaming protocol

[sunng87]

Grpc streaming rpc is a fun idea for doing rpc.

The client may send a few continuous requests segments before it asking server to response. And vice versa for the server. It's also possible to do it bidirectional.

This requires an upgrade of our wire protocol.

The API requires some design too. We may use a special lazy-seq to do it transparently. And helpers will be needed to create such kind of lazy-seq.

Another idea is to use core.async chans for streaming data.