Inbound message stream handle for reading a sequence of messages from the peer.

• Server side (client-streaming and bidirectional-streaming RPCs): Reader<Request> reads the stream of requests sent by the client.
• Client side (server-streaming and bidirectional-streaming RPCs): Reader<Response> reads the stream of responses sent by the server.

Call Read() in a loop until the returned ReadFuture resolves to kEndOfStream, which indicates that the peer called Finish() (or dropped its Writer) and no more messages will arrive.

Destroying a Reader before the stream ends stops reading incoming messages (any unread or subsequent messages from the peer are discarded).

• Server side: it does not cancel the call or affect the outbound Writer or UnaryWriter.
• Client side: if the RPC has not completed yet, it cancels the call, as Cancel() does, since nothing is left to observe its outcome. This keeps the server from running an abandoned RPC, such as an unbounded server stream, until the connection closes.