For #540 - Refactor http code for reuse

[lars-t-hansen]

This splits a bunch of logic out of the Kafka code into a "background uploader" idea and a "http post" idea. Functionality-wise this should represent no change. I will need to test some more before landing but otherwise this is done. Once it's done, #540 is easy.

[lars-t-hansen]

Despite --silent and --retry, curl spams a lot on stderr when it can't contact the server. Sonar handles this but not very gracefully, it will dump a debug message every time it can read something, it does not wait for a complete error message, so sometimes there can be lots of debug spew for what's one line from curl. This is more or less by design since not all programs write complete lines but since it's curl I guess we could assume there will be a newline eventually. Since the problem is not new I'll file a followup, not fix it in this patch.