MINOR: server: add a list of safe, already reused idle connections

These ones are considered safe as they have already been reused.
They will be useful in "aggressive" and "always" http-reuse modes
in order to place the first request of a connection with the least
risk.
diff --git a/include/types/server.h b/include/types/server.h
index 411ea4b..cbeead9 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -168,6 +168,7 @@
 	struct list actconns;			/* active connections */
 	struct list priv_conns;			/* private idle connections attached to stream interfaces */
 	struct list idle_conns;			/* sharable idle connections attached or not to a stream interface */
+	struct list safe_conns;			/* safe idle connections attached to stream interfaces, shared */
 	struct task *warmup;                    /* the task dedicated to the warmup when slowstart is set */
 
 	struct conn_src conn_src;               /* connection source settings */