Stream upload facilities

[eloylp]

Problem

Goomerang its currently using protocol buffers as message serialization mechanism. That implies when sending/receiving messages, we need to load them entirely in memory. There are also limits in terms of sizes for proto message specification, being currently the maximum size per message around 2 GB.

In most of the use cases, proto messages are very lightweight and they only carry little application data. But there are cases like uploads that would benefit from an streaming protocol.

Goals

Research and provide to the users of the library a high level streaming protocol, built on the top of the current library facilities. It should be able to split large messages in chunks. We should keep in the equation the standard library. It should play well with the io.Reader and io.Writer Go interfaces.

This feature would cover a very specific use case, so it should probably not be part of the main client and server code.