BUG/MINOR: log: fix request flags when keep-alive is enabled

Colin Ingarfield reported some unexplainable flags in the logs.
For example, a "LR" termination state was set on a request which was forwarded
to a server, where "LR" means that the request should have been handled
internally by haproxy.

This case happens when at least client side keep-alive is enabled. Next
requests in the connection will inherit the flags from the previous request.

2 fields are impacted : "termination_state" and "Tt" in the timing events,
where a "+" can be added, when a previous request was redispatched.

This is not critical for the service itself but can confuse troubleshooting.

The fix must be backported to 1.5 and 1.4.
(cherry picked from commit a83a50bd7d9389d3f799b9e3b7c7be346be3fab7)