MINOR: session: Add the idle duration field into the session

The idle duration between two streams is added to the session structure. It
is not necessarily pertinent on all protocols. In fact, it is only defined
for H1 connections. It is the duration between two H1 transactions. But the
.get_cs_info() callback function on the multiplexers only exists because
this duration is missing at the session level. So it is a simplification
opportunity for a really low cost.

To reduce the cost, a hole in the session structure is filled by moving
.srv_list field at the end of the structure.
diff --git a/src/session.c b/src/session.c
index 710b6b6..07828df 100644
--- a/src/session.c
+++ b/src/session.c
@@ -50,6 +50,7 @@
 		vars_init(&sess->vars, SCOPE_SESS);
 		sess->task = NULL;
 		sess->t_handshake = -1; /* handshake not done yet */
+		sess->t_idle = -1;
 		_HA_ATOMIC_ADD(&totalconn, 1);
 		_HA_ATOMIC_ADD(&jobs, 1);
 		LIST_INIT(&sess->srv_list);