MINOR: session: implement session_free() and use it everywhere

We want to call this one everywhere we have to kill a session so
that future parts we move to the session can be released from there.
diff --git a/include/proto/session.h b/include/proto/session.h
index 79589d3..487f318 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -31,6 +31,7 @@
 #include <types/session.h>
 
 extern struct pool_head *pool2_session;
+void session_free(struct session *sess);
 int init_session();
 
 #endif /* _PROTO_SESSION_H */