Add support of watching events to Connection

[ondrej-fabry]

This PR adds support for event watching to Connection (Stream API). This initial draft proposal was done by @edwarnicke in #42. However, this PR does not change the generated RPC code in any way for now.

Quick look at the new API:

type Connection {
	// ...
	WatchEvent(ctx context.Context, event Message) (Watcher, error)
}

type Watcher interface {
	Events() <-chan Message
	Close()
}

See the stream-client example for usage details.