Streaming calls do not time out

[gaschler]

Clients should always set a deadline: https://grpc.io/blog/deadlines
Client should support setting a timeout and provide a default.
Currently, Client::Write blocks without a time limit (for instance if the server address is wrong).

[gaschler]

With #31, unary calls now offer a timeout (without default). Renaming the issue.

Unfortunately, for streaming rpc, set_deadline is valid for the entire stream, so implementing a timeout that is holds for each Write will require a large work-around.