When http_reply events are received before http_request events, either
through faking traffic or possible re-ordering, it is possible to trigger
unbounded state growth due to later http_requests never being matched
again with responses.
Prevent this by synchronizing request/response counters when late
requests come in.
Also forcefully flush pending requests when http_replies are never
observed either due to the analyzer having been disabled or because
half-duplex traffic.
Fixes#1705
af1714853f