MAJOR: sessions: Store multiple outgoing connections in the session.

Instead of just storing the last connection in the session, store all of
the connections, for at most MAX_SRV_LIST (currently 5) targets.
That way we can do keepalive on more than 1 outgoing connection when the
client uses HTTP/2.
diff --git a/include/types/connection.h b/include/types/connection.h
index dbf985b..85afca0 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -414,7 +414,7 @@
 	struct wait_event *send_wait; /* Task to wake when we're ready to send */
 	struct wait_event *recv_wait; /* Task to wake when we're ready to recv */
 	struct list list;             /* attach point to various connection lists (idle, ...) */
-	struct list session_list;     /* List of all sessions attached to this connection */
+	struct list session_list;     /* List of attached connections to a session */
 	int xprt_st;                  /* transport layer state, initialized to zero */
 	int tmp_early_data;           /* 1st byte of early data, if any */
 	int sent_early_data;          /* Amount of early data we sent so far */
diff --git a/include/types/session.h b/include/types/session.h
index e0e1455..334a071 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -37,6 +37,13 @@
 #include <types/task.h>
 #include <types/vars.h>
 
+struct sess_srv_list {
+	void *target;
+	struct list list;
+};
+
+#define MAX_SRV_LIST	5
+
 struct session {
 	struct proxy *fe;               /* the proxy this session depends on for the client side */
 	struct listener *listener;      /* the listener by which the request arrived */
@@ -47,8 +54,7 @@
 	struct vars vars;               /* list of variables for the session scope. */
 	struct task *task;              /* handshake timeout processing */
 	long t_handshake;               /* handshake duration, -1 = not completed */
-	struct connection *srv_conn;    /* Server connection we last used */
-	struct list conn_list;          /* List element for the session list in each connection */
+	struct sess_srv_list srv_list[MAX_SRV_LIST]; /* List of servers and the connections the session is currently responsible for */
 };
 
 #endif /* _TYPES_SESSION_H */