An order tag can be added to a message or a response to indicate
whether it should be sent after or before other messages with order tags
referencing a same stream
A stream is an opaque identifier that defines a set of messages
or responses that are ordered wrt one another using to order tags.
The Req is a helper object used to create requests and attach them
a stream of data. If the stream is a fixed Bytes and not a ByteStream,
Req is cheaply clonable to allow the request to be sent to different
peers (Clone will panic if the stream is a ByteStream).
The Resp represents a full response from a RPC that may have
an attached stream.