Stream policy

[arkstack-dev]

§3.3 forbids server-initiated bidirectional streams.
§3.4 reserves unidirectional streams entirely (both directions) for v0.2+.

Open question: is anything useful being foreclosed by this?

Candidates I can think of, all currently impossible under v0.1:

• Server pushing NOTIFY payloads without occupying the session stream's
  response slot (likely v0.2 candidate, Appendix A item 3)
• Server-side streaming of large result sets on a separate stream so the
  session stream stays available for control messages
• WAL streaming / logical replication as an out-of-band channel on a
  unidirectional stream

If you have a use case that needs server-initiated streams and can't be expressed as client polling or as a client-opened stream that the server writes into, this is the thread to flag it. I'd rather hear about it now than ship v1.0 and discover the policy was too restrictive.