REORG: include: move session.h to haproxy/session{,-t}.h

Almost no change was needed beyond a little bit of reordering of the
types file and adjustments to use session-t instead of session at a
few places.
diff --git a/src/peers.c b/src/peers.c
index 45e438d..f576eaa 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -27,6 +27,7 @@
 #include <haproxy/obj_type-t.h>
 #include <haproxy/peers.h>
 #include <haproxy/task.h>
+#include <haproxy/session-t.h>
 #include <haproxy/signal.h>
 #include <haproxy/time.h>
 #include <haproxy/tools.h>
@@ -41,7 +42,6 @@
 #include <haproxy/fd.h>
 #include <proto/log.h>
 #include <proto/proxy.h>
-#include <proto/session.h>
 #include <proto/stream.h>
 #include <haproxy/stick_table.h>
 #include <proto/stream_interface.h>