Application-level Producer API

[cararemixed]

Above the lower level apis, we should provide a managed producer API. This will handle concerns such as connection multiplexing, topic metadata refresh (and creation if enabled), batching single messages into message sets, and error handling (retry, back off, throttling).

The design will be done from the application's point of view. We'll start by building up some module signatures and go from there. The implementation will try to align with the mechanisms we already use (like AsyncSeq and DVar?).