server: Flush headers when using wait=true and stream=true

Many http clients will missbehave unless they get an initial http-
response, even when long-polling. It also saves the user/client from
having to handle headers on the first action of the watch, but rather
handle the response immediately.
Merge pull request #877 from Rawn/flush-headers-on-wait-stream

server: Flush headers when using wait=true and stream=true