allow non-streaming requests over PUSH/SUB

[jkarneges]

When using the PUSH/SUB interface, handling HTTP requests that last longer than 60 seconds requires session management (remembering request IDs and sending keep alives to Zurl). It might be nice to optionally make the PUSH/SUB interface behave similar to REQ/REP, where there is only a single message for a request and a single message for a response, and no session management to worry about.