[stream-core] Improve consistency of use of non-blocking calls

[badgerwithagun]

A few improvements to use of RX in the core streaming API.

• Support non-blocking rate limit acquisition
• Allow callers to hook into completion of the (already non-blocking) sending of socket messages.
• Fix (unobserved) bug where connection emitters were fired before the socket was ready

Still being tested. Presented as draft for comment.

[gewure]

any update?

[walec51]

is there any progress on this PR?

if not I will have to close it