[MINOR] http: logs must report persistent connections to down servers

[MINOR] http: logs must report persistent connections to down servers

When using "option persist" or "force-persist", we want to know from the
logs if the cookie referenced a valid server or a down server. Till here
the flag reported a valid server even if the server was down, which is
misleading. Now we correctly report that the requested server was down.
We can typically see "--DI" when using "option persist" with redispatch,
ad "SCDN" when using force-persist on a down server.
(cherry picked from commit 2a6d88dafe5e5628abb962caf395143d0c0b6024)